Many articles and studies will tell that too many case fans is a bad thing. The ideal air flow setup is one intake and two exhaust (one being the PSU). But there are times that you will need to add an extra case fan.
You will need a power drill and a hole saw. For a 120mm fan you will need a 4-3/8 inch or 4-1/2 saw depending on your mood. For an 80mm fan you will need a 3-1/8 inch saw or a 3 inch depending how you like it. For a 92mm you will want a 3-1/2 inch saw. You may also want a fan grill to keep loose wires and such from hurting the fan. I used a 2-1/8 inch saw on this project.
If you are planning on doing this on a thin case side panel, I suggest doing this on a work bench to prevent bending.
And that’s it. It took me 10 minutes to put in the fan. Don’t forget to vacuum to shavings up. If you wondering where to get the tools check out http://www.crazypc.com/.