Mount a Motherboard in a Case

This article tells you the basics of mounting a motherboard in a computer case.

Steps

Verify that you are as static free as possible.
Open the case you want to mount the motherboard in.
Verify all metal hex nuts are in place to fit the new board. Make sure none will short any solder points on the board.
Verify all plastic spacers are in place to accommodate the new motherboard.
Place the motherboard into the case.
Screw the screws in place to hold the board in place. Make sure they are the only thing that line up with the hex nuts...That is the whole purpose of those.

With computer hardware, you need to stay grounded.
Don't twist, warp, or flex the motherboard - this can cause permanent damage.
It is best to have all the screws in place and make sure that the motherboard is properly aligned before you tighten down the screws.
Be very careful when placing the motherboard on the hex screws as you can cut or nick one of the circuit traces on the motherboard causing permanent damage.
Before you do this, you can get rid of your static electricity by touching unpainted metal for ten seconds. Try using the power supply for this. It is best to hold onto that with your other hand while doing the whole process.
Do not over tighten screws doing so can crack the motherboard, tighten them far enough so that with one wrist turn of the screwdriver the screws start to remove.

Avoid working on carpeting as it generates static electricity.
Make sure your hands are dry.
Use an anti-static strap that is electrically grounded. Straps can be purchased at electronic supply stores and many computer parts dealers.
Do not use a magnetic screwdriver.
There should only be as many hex nuts as there are screw holes to hold the board in place
At no time should you ever leave the motherboard/CPU/other components on the ground or on the table. Most people rest their motherboard on the box it came in. The anti-static bag outside layer is not conductive and is made of the same material as inside the bag so they are a good place to put your raw motherboard.
Do not wear fleece clothing.