I'm going to keep this one fairly simple, and it shouldn't be to hard to follow.
Necessary Materials:
Case
Case Feet
Drill and Bits
Safety Glasses
Method of attachment (ex: bolts)
Optional Materials:
Spacers (for going between the feet and the case)
Nail/Hole Punch
Now to start off with, there are only three reasons to even do this mod in the first place. A) For aesthetics, to make the case look better. B) Depending on what case feet you are using, widen the base and increase the stability of the case. C) To lift the case up for the addition of a bottom fan.
You should consider these things carefully before you begin modding. The most essential, yet probably the most boring part of any mod is the planning stage.
OK, now let's set up our work area:
I'll be using a case which I have already begun to mod. The chassis is from an old Pentium II Compaq Presario. I pulled the case feet off of a decade old computer my neighbor was about to take to the dump. Most likely, your case feet will look something like this, but that doesn't change the mod much.
Decide exactly where you want your feet to go:
I decided I wanted my feet flush with the back of the case, so I adjusted accordingly. But those rubber feet are going to have to come off. They look like they might make good spacers which may help to further lift the case.
Mark your holes off. A sharpie works well on metal. Now we've reached the stage where the real modding is about the begin. But you must be careful, if you have a steel case, a single tiny flake of steel could literally cause your eye to rust. Trust me I know. It happened to my brother. The doctor had to dig out a portion of his cornea to stop the spread of the rusting.
So, lets get serious about safety. I look like the grim reaper in these pictures, but whatever.
Select an appropriate bit size.
Or maybe one of these would be better.
Before you begin drilling, to make life easier, you can use a nail punch to create a divot for the drill bit to rest in so that when you start drilling the bit won't wander.
Double check your positioning, and drill away.
Repeat for however many feet you have. Those rubber feet that I peeled off came in handy. I drilled a hole in the center and put them between the case and the feet to lift the case up even more. It may also reduce vibration in the case a little bit.
I attached my feet with simple bolts and wing nuts, but if you bought case feet, they probably came with their own way of mounting.
If you notice any errors or have something to add, PM me and I'll add it.