This is not my idea, I read it in Maximum PC February 2001. Get a hole cutter from hardware store and make sure it has a pilot bit.
1. first off proceed at your own risk. If it is not your computer ( parents) ask first!!!
2. If you are cutting anywhere else but the removable side panels, remove everything from case including the power supply. You don't want all the metal bits on your new motherboard
3.Determine where you want to have the fan mounted. ( good idea to have blowing @ cpu)
Draw square to determine the fan's intended location. Mark from one corner to the other to locate the center.
4.Secure side panel or case to keep it bouncing around, you don't want scuff marks all over your case from slipping. I used 3 1/8 for 80mm hole. Cut a circle into the center of the square.
5. Use dremmel tool to clean up rough edges. Position a fan over the hole and mark where the screws will be mounted. Drill holes for screws.
6. Mount the fan and the guard on the panel/case. Make sure the wiring is long enough that the side panel can be opened and closed. Connect the wiring to power supply connector inside the case.
633 Celeron @1055, Abit bx-133, 256 pc 133 @ cas 2-2-3, ATI radeon 64 mb DDR