I did this, and it was quite easy.
First, get the fan you want to install. athe size of the fan will be determined by the amount of space you have between your PSU and your first optical drive. I could only fit a 80mm fan, so depending on your case (I have a raidmax) will determine if you can even do it at all, or if you can fit 92mm or more.
Second, take the fan down to your local hardware store and find a hole saw that is slightly smaller than the diamter of the portion of the fan that spins.
Third, take everything out of your case - absolutely everything, because of the small metal peices that will fall in there - and use painters tape acroos the top of the case so you don't scratch the top. Measure out exactly where the hole is going to go, and prill a small piot hole in the center of your circle for the hole saw. I had to use a corded drill FYI, my cordless didn't have enough power to punch through the case.
Fourth, measure where hte screw holes go by placing the fan on top of the case where the hole is, and drill those in.
Fifth (optional) is take a dremmel and smooth out the rough edges from your drilling, and use the buffer attachment too. Get somehigh gloss metal spraypaint that matches your case color and fix any blemishes/scratches to hide your f/ups.
Last, vacuum out the case, place everthing back in, and enjoy. I suggest getting a nice fan grill and a LED fan so when you turn on your case you can look at your mod with pride