I've done it, it not that hard. This first time is always the worst though.
First thing you need is a diamond tipped hole saw. A variable speed drill, some clamps, glass of water and a piece of wood.
You use the wood as a template. 1x6 works great, drill a hole slightly bigger that the hole saw bit. Then you clamp the template to the tank where you want your hole. Pour some water in the hole in the template and start drilling. You need to drill slowly with little pressure. About 600-800 rpm is good and keep the glass wet. It will take a while but there is no rushing the drill.
I would suggest using a old tank or piece of glass before attempting on a expensive aquarium. Also Tempered Glass can not be drilled.
EDIT: Most LFS will drill tanks in my area. It is around $10-15 a hole so it can get expensive if you want a holey tank