Now that you have selected your parts, you get to what is arguably the more fun part of the process: putting the parts together.
Tools and equipment
There are a few tools that you will need to begin assembling your computer. The following is a good starting point, as they include the bare minimum to build a working computer.
Before you begin building or refitting a computer, you will need some basic tools:
#2 phillips-head (cross-shaped) screwdriver
All available manuals for every component to be installed
Anti-static Wrist Strap (optional)
Grounding Mat or anti-static carpet (optional)
Optional, but useful tools
Some other tools and equipment can come in handy as well, such as:
Spring action parts grabber.
A second, working computer to swap parts, look for tips, ask for help online, download drivers and patches, etc. - very useful
A can of compressed air - useful when working with older parts that have collected dust. A better alternative but also more costly, is a vacuum cleaner designed for cleaning electronics.
Before you begin poking around inside your computer have the proper mindset. Read the instructions that came with your hardware. Do not work inside your computer if you are in a hurry or do not have the proper level of knowledge to complete the work. If you do not know how to do something reread the instructions, find futher information online, or simply ask some one who has more experience.
Find a dry, well-ventilated place to do your work. You should have plenty of light to see your components with, and if possible, you should choose an area without carpet on the floor, as carpet tends to attract a lot of static, and most of this can be averted by choosing a conductive floor surface. An unfurnished basement is a perfect work location.!
To assemble your components, you will need a basic toolkit. For this kit, you will need
a Phillips-head (cross-shaped) screwdriver
an anti-static wrist strap, for grounding yourself with (these can be purchased at your local electronic supply store) - this can often be a huge saving over the amount of money that you waste by destroying your components with static electricity. Do not be tempted to connect yourself directly to a tap or other grounded object with only a length of copper wire, if the machine is still plugged in and there is an electrical fault, the consequences could be fatal. Anti-static wrist straps have a high resistance, thus limiting any potential current flowing through your body to safe levels.
Unpack all the components, and put them on top of the anti-static bags they came in, as this will prevent stray static from reaching your components. (Note. Anti-Static bags conduct electricity on the external surfaces!) Make sure you read the following section, as it contains some important safety information.
Dismantling electronic components such as your Power Supply or Monitor is an extremely dangerous thing to do - do not do it! They contain several high-voltage exposed components, and can cause you severe electric shock if you touch them, even when the unit is not plugged in. So, please, don't even think of trying.
Nobody except you is at fault if you shock your components with static electricity. Make sure that you take the precautions in the previous paragraph to ground yourself from static electricity, such as with an anti-static wrist strap or grounding mat. (Note: if you really must work on a computer and haven't got proper anti-static equipment, it is usually OK if you make sure that you don't move about much; are not wearing any static-prone clothing; handle components by the edges; and regularly (once a minute or so), touch a grounded object.). The case metal of your PC's power supply will usually be a suitable grounded object. As noted above, touch it every few minutes while you are working on your PC.
Please construct your computer in a dust free enviroment! The worst thing for components (besides static) is overheating, most commonly caused by fans and heatsinks clogged with dust. If this occurs, the fans will fail, and will cause your system to overheat and fail. However, if they are clogged, just buy a can of compressed air and blast the fans with it to clean them.
Unplug your PC's Power Supply before installing or removing any components - if power is flowing to components as they are installed or removed, they can be seriously damaged.
Never cut the grounding pin off your power cord. This "safety ground" stands between you and potentially lethal voltages inside the power supply.
Be wary of sharp edges! Many lower-end PC cases have sharp, unfinished edges. This is especially so on interior surfaces, and where the case has been cut or punched-out. Use care and take your time to avoid cutting your hands.
Start by putting your case down on your work surface, with the side opposite of the case door facing down, and open the case.
Try to find the motherboard standoffs that should have come with the case. They are screws with screw holes on the top.
Insert the standoffs into the holes on the motherboard plate in the case that correspond with the holes on your motherboard and screw them into the bottom of the case. Also remove the I/O Shield that came with the case, and put in the I/O Shield that came with your motherboard.
Some case styles make it difficult to install the motherboard or the CPU with the power supply installed. If the power supply is in your way, now would be a good time to take it out and set it aside (we'll put it back in later).
Insert the motherboard by placing it into the open case. The ports in the upper left hand corner should line up with the I/O Shield and go through the holes in it. The screw holes should line up with the motherboard standoffs. Put a screw into each hole in the motherboard with a hole below it.
Now that you have your motherboard in, you may plug in the other components.
An example of a CPU socket, Socket AAs installation of the different makes of CPU can differ between brands, it is generally safer to refer to the manufacturer's instructions, that are provided with the CPU. If you are using a thermal paste with your CPU, follow the directions that came with them for details on how to apply it.
The two things that go wrong the most often and most expensively (minimum of a killed CPU, sometimes more) in building one's own computer are both related to the CPU and its cooler:
Switching the computer on "just to see if it works" before adding any CPU cooling unit. Without cooling, CPUs heat up at extreme rates (a CPU heats up anywhere between ten times and a thousand times as fast as a cooking area on your stove!) By the time you will get the first display on the screen, your CPU will already be severely overheating and might be damaged beyond repair.
Mounting the CPU cooler improperly. Read the instructions that came with your CPU and cooler very carefully and ensure you are using all components in the correct order and correct place.
Make sure you get a cooler that is compatible with the CPU you have. Most brands come with multiple mounting brackets that will suit all different chipsets, but it's best to check for compatibility just in case.
If using thermal paste, apply it only to the CPU die (the square piece of silicon in the middle of the CPU) and do so sparingly -- most modern CPUs take no more than a grain of rice sized dab of thermal paste. Some people do like to wipe some onto the heatsink's surface and then wipe it smoothly off so that bits of it may get into tiny holes for better heat transfer. See Arctic Silver Instructions for more info on how to apply and remove thermal paste/grease. (It was written to be specifically for Arctic Silver paste, but the same techniques can be applied to other brands of thermal paste.)
If using a thermal pad supplied with your cooler, make sure you remove any protective tape just before installing and do not get it dirty - and do not combine thermal pads with thermal paste, it's either one or the other. Then, check that you install the cooler in the right orientation and that you set it flat on the CPU die without exerting undue pressure on any edges or corners - the latter can make small pieces of the die break off, killing the CPU.
One option you may consider, before installing the heatsink, is to "lap" the heatsink, which means to smooth out the bottom surface. To do this, start by sanding in smooth circular motions with a coarse grain sandpaper to smooth out the worst of the uneveness, then, as it starts to get smoother, switch to a finer grained sandpaper (the numbers go up as the sandpaper is finer, so something such as 60 is coarse while 220 will be very fine.) If you get it right, it should have a surface which feels completely smooth to the touch where you can almost see a reflection in it. Some companies producing heatsinks lap the surface themselves and this will be unnecessary, but, it is very rare. A lapped heatsink is far more effective due to having better contact with the chip.
Tighten the cooler using only the specified holding devices - if you did everything right, they will fit. If they don't fit, check your setup - most likely something is wrong. After mounting the cooler, connect any power cables for the fan that is attached onto the cooler. Then, if everything is seated tightly and firmly, you can safely run your first test, making sure the CPU fan does run (you have a few seconds of safety margin here, but if the fan is not running, switch off the system quickly, and then check your cabling.)
As an aside to the instructions above, it has been my personal experience that fitting the CPU and heat sink is best done on a supportive surface (a telephone directory on a table in my case) prior to installation, to avoid excessive flexing of the motherboard.
Next, you will need to install your RAM (random access memory). Start by pushing on the levers on both sides of the DIMM socket, so that they move to the sides. Do not force them too hard.
Put the RAM module in the socket. Line up the notch in the center of the module with the one in the center of the RAM socket, making sure to insert it the right way. Force the module until both levers go into the notches on both sides of the module. Although this does require a fair bit of force, do not over do it or you may break the RAM module.
Start adding RAM at "Bank 0" or "DIMM 1". If you don't start from "Bank 0" or "DIMM 1" the system will think there is no RAM and not boot.
On newer motherboards with 4 slots, you'll see alternating colors. For example, slot 1 is blue, slot 2 is black, slot 3 is blue, slot 4 is black.
If you were to put 1 gigabyte of RAM in your PC, it's best to use dual channel 512MBx2 chips. Put the first 512MB chip in slot 1, and put the 2nd chip in slot 3 (the two slots that are blue) - leaving slot 2 empty. This will give you better performance, vs. putting 1GB in slot 1, or two 512MB chips in slot 1 and 2.
Some cases include power supply units (PSUs) that are already installed. Some of these are underpowered or of lower quality, especially in inexpensive case/PSU bundles. Before purchasing or installing any PSU, make sure that the supplied wattage is sufficient for your components. Power requirements are usually listed in the manuals that came with your components. It is important to note a power supply's total power, and the power at each voltage: 3.3, 5 and 12V. If any of these do not meet your requirements, the rest of the specifications don't matter.
Some companies have calculators to help you determine what your power supply needs are; if you are the type to just plug in the numbers without reading the details, you should buy a power supply that is 1.5 to 2 times the wattage that results from these calculators.
Power Supply Calculators
The JourneySystems Power Supply Calculator http://www.journeysystems.com/power_supply_calculator.php
The eXtreme Power Supply Calculator v2.0 http://www.extreme.outervision.com/psucalculator.jsp
If your mother board has a built-in video adapter you want to use, skip this section.
If you have an AGP video card: Install the video card into the AGP socket. This is always the top expansion slot near the back of the computer. AGP slots are often brown, but can also be strange colours such as fluorescent green. Check the motherboard for levers (or similar devices) that are part of the AGP slot to help hold the card in place. These must be retracted before insertion of the card. Check the motherboard's manual for information on how to use these devices (if your motherboard has one.) Push the card into the socket (AGP slots are often pretty tight, don't be afraid to push it until it's well inserted), then screw it in at the top of the metal bracket. If it has a power connector, connect it to a 4-pin molex connector. If it has a pass through, do not connect it to a hard drive.
If you have a PCI Express video card: Put this in the same way as a AGP video card, however the slot where it goes looks a little different than the AGP slot. It will look like it but have an extra spot on the slot as opposed to the 2 slot parts on an AGP slot. PCI Express is 16x meaning it can handle the transfer of twice as much data as AGP 8x.
Installing drive jumpers
Each SATA connector can only handle one drive, so there is no need to adjust jumpers -- you can skip this section. Before you install IDE/ATA drives, you will need to set the drives jumpers. Each IDE/ATA channel can handle two drives, a master and a slave. Consult your motherboard/drive's instructions on how to set the jumpers. The jumper configurations are usually either printed on the back, or on the top of the drive. Drives can be configured in 2 ways: Drive Select or Cable Select.
"Cable select": Use this if you have 80-pin cables. Cable select automatically assigns slave/master based on the plug on the IDE cable the drive is plugged into. Put the jumper on CS.
"Drive select": If you are using a 40 pin cable, you must use "drive select". master/slave is determined by the jumper. In this mode, configure one of the drives the master, and the other slave. If the IDE channel has only one drive, check your motherboard documentation for the appropriate setting, which is usually master.
Next install the hard drive and optical drives.
How a drive is installed will depend on the case.
When using an IDE cable, plug the two connectors that are closer together into the 2 drives, and the third to the controller or motherboard. The connector furthest from the board should be attached to the Master drive. Make sure the drive that you will install your OS on is primary master. (Note: IDE connectors are keyed, so it should be impossible to insert them backwards. However, it doesn't require very much force to do this, and you may destroy your motherboard if so. Also look carefully at the drive and the cable connection before you try to connect them. You will probably see a "missing" pin on the drive, and a corresponding blocked socket on the connector. If you break a pin on the drive, you will probably have a worthless drive. Note: most parallel IDE cables have a colored stripe down one side. That colored stripe signifies "pin 1" - and usually will line up next to the molex power connection on your drive. Use this rule of thumb if your connectors aren't keyed.) Next, plug a 4 pin molex power connector into each hard drive and optical drive. If you are installing the power connector to a SATA drive, only install one of the power connectors. Some drives have the option of using either the SATA power connector (a flat about 1" wide connector) or the standard molex connector; use one or the other, not both. Make sure the IDE cables are in the proper slots - usually, you should mount the master drive in slot 1, and any slave drives in the other IDE slots. For better data transfer, you can purchase heat-protected high-end data cables at your nearest electronics store.
If you install a floppy disk drive, the cable is very similar to the IDE cable, but with fewer wires -- and a strange little twist in the middle [ need photo here ]. Floppy drives do not have master/slave. BUT the floppy disk connector is not usually keyed, making it all too easy to plug it in the wrong way! One wire in the IDE cable will be coloured differently: this is pin 1. There is usually some indication on the floppy drive as to which side this is. The power plug for a floppy is 4 pins in a line, but rather smaller than the standard hard drive power connector. Plug the end of the cable with the twist into the floppy drive ("drive A:"). Plug the other end of the floppy ribbon cable into the motherboard. If you install a second floppy drives, plug the middle connector into "drive B:". The twist between drive A: (on the end) and drive B (in the middle) helps the computer distinguish between them. 
Expansions and connections
Now, install any PCI cards that you have. These generally include sound cards, network cards, video cards, and TV tuners. These fit into the white slots that are just below your AGP/PCI-E slot (if your motherboard has one).
Also plug in any power cords, including the 20pin and the 4 pin cords that you haven't plugged in yet. Before you finish up and power it up, you need to connect the power/reset buttons and front panel lights. The plugs from the front of the case will be labeled. The pins on the motherboard may be labeled, but they will be difficult to read because they are small. The foldout that came with the motherboard tells where to connect these connectors. The front panel LEDs are polarised: usually the positive wire is white.
In addition, you can optionally connect any case-specific port if it is supported by the motherboard. Some common examples include USB ports and sound ports.
Next, close the case and take your computer to where you will be using it. Plug in the power cord, mouse, keyboard, monitor, and any other peripherals you may have to the computer.
Press the power button. If smoke appears (it shouldn't, unless your power supply or cooling systems are insufficient), or if the computer doesn't do anything, unplug the power cord immediately and check the steps above to make sure you haven't missed anything. Give special attention to the cables and power connections. If the computer does appear to come on, but, you hear beeps, listen carefully to the beeps, turn the computer off, and refer to your motherboard's manual for the meaning of the beeps. Some boards have an optional diagnostic device, usually a collection of LEDs, which when properly plugged in will inform you of the nature of the problem. Instructions for installing this as well as the meaning of it's display should be in the manual for the motherboard. If it turns on but the only thing that comes on is your power supply, turn off your computer. This probably means something is shorted, and leaving it on could damage the parts.
At this point, you will wish to set certain options in the Computer's BIOS (usually by pressing 'F1' or 'Del' a few seconds after boot.) These options will be explained in the motherboard manual. In general, the default options are OK, but you may wish to set the computer's hardware clock to the correct time and date. The BIOS is also where you determine the default boot order of the system, typically Floppy, then CD-ROM, then Hard Disc.
If you want a further quick test, before you install an operating system, you may find a bootable CD-ROM such as Knoppix extremely useful.
Comparison of VGA and DVIYou may also need to install peripherals such as keyboards, mice, speakers, monitors and extra devices like printers. Each of these comes with a connection cable and software to control them (driver). There are two standard connectors for mice and keyboards; PS/2 connectors and the more modern USB connectors. (Look at the images to see which one yours are) . Plug the mouse and keyboard in the appropriate slot.
Note: If you intend to install an operating system from a boot CD or floppy, or modify bios settings you will need to use either a PS/2 keyboard, a USB to PS/2 converter, or a motherboard that supports USB devices. Otherwise your keyboard will not work until the operating system has loaded USB drivers.
To plug in speakers, you need a sound card, which is preinstalled in most motherboards. The speaker plug will probably be a standard 3.5mm plug, just like headphone plugs. Since speaker sockets and microphone sockets are the same type, they are often colour coded. Green usually represents speakers and pink represents microphones. In most cases, a third plug is present - this is the input channel which allows the sound card to receive an audio signal from another device, such as a Hi-Fi system.
Monitors will either have a VGA or a newer DVI plug (see picture, as they are a lot less apparent than PS/2 / USB comparision). Most monitors use VGA connectors, and so most graphics cards have VGA output. If you have one type of plug and the graphics card has another, you can easily buy an adapter.
Most other devices use USB connectors, so follow the instructions on the CD you were given with them.