Install all of your expansion cards and attach them with the screws you removed earlier. If the slots are a little tight, you might put slight downward pressure on the front edge of the card (the edge furthest from the opening in the back of the case) until it starts in, then push the back edge in. Avoid wiggling the card from side to side or applying too much pressure onto the motherboard by pushing too hard on the top of the card.
If you have a lot of expansion cards or a limited number of expansion slots, you may not have a place to attach the brackets for your parallel and serial ports. To remedy this, most cases have punch-outs that allow you to remove the parallel and serial connectors from the bracket and install them directly to the back of the case. You are now ready to connect the power supply to the motherboard.
AT style power supplies have two 6-pin plugs labeled P8 and P9 that are installed with the black wires meeting in the center. If you install them the wrong way, your motherboard will not power up (but will usually not cause any damage - though I cannot guarantee that!). In most cases the socket will have a small groove behind particular pins, which should match up with a small tab on the plug. You need to tilt the plug slightly back so the tab fits into the groove, straighten it up and push down to install it.
ATX power supplies have a single 20-pin connector that can only be installed one way. Line up the tabs on both the socket and plug, and push down until it snaps into place.
Your external devices can now be connected, such as the mouse, keyboard and monitor. It is a good idea to connect only what is necessary to test the installation, and to leave the case open in the event that you need to make some adjustments.
Plug in the power cord and turn on the system. If everything has been connected properly the motherboard will run its POST routine (Power On Self-Test) to check the memory and identify certain devices. During POST, you will usually see a message indicating how to enter the CMOS setup (typically the DEL key), which is where the system maintains the basic configuration information.
Upon entering CMOS setup, a menu will be displayed with various options. The 'Standard CMOS Setup' option is where you will adjust the date and time and identify the possible boot devices. In most cases you should set the hard drives to 'Auto Detect' and verify the settings for the floppy drives. If you don't have a B: drive, set the value to 'Not Installed'. Hitting the ESC key will usually take you back to the main menu.