It can be done with minimal tools. Like a sharp nail(as a centerpunch) a hard-heeled shoe(as a hammer) a 3/8" power drill with a 1/8" bit(get two of them to drill a hole for a 120mm fan-and a plug-in drill is better than a cordless for this task. Add a small common, flat blade screwdriver(as a chisel), and a half-round as well as a round needle file.
You need to mark the circle for the fan. Then drill 1/8" holes around the circumference of the hole. Out of round 60, 80, 92 or 120mm fan holes really catch your eye, so the holes must be centered on the line. Just drill them as close together as you can, while preventing the drill bit from wandering into the hole you drilled just before this one. You'll get the spacing right after a little practice.
When the holes around the circle are finished, just use the "hammer" and the "chisel" to break the "spider web" that holds the center of the hole in place. Voila. You'll have a jagged-edged hole that can be made smooth with the half-round file. And a shiriken of little value.
Mark and drill the four mounting screw holes for the fan. They'll need to be made a bit larger, this is where the small round needle file is a must.
Sheet steel ~1mm thick is easy to dimple and distort. And APITA to drill. If you can use a case cover from a defunct case for practice it's a good idea.