Search on the net, there is tons of how-to's out there, or go to ars and search there.
Some few things, get a good cutting tool, either a dremel which is good for small ones, or a holesaw. Use tape or something to mask both sides, and get a good block of something under it so the metal dont bend at the edges. Cut slowly letting the wheel speed/sharpness do the work, not pressure. Clean up the edges after that. Watch for heat so it doesnt discolour coating and edges of hole if you aint gonna cover it with something.
Those are some few things that i remember seeing.