best thing i did when drilling my 30gal was i took some scrap plexiglass and drilled the same size hole in it first used silicone to hold it in place on the outside of the tank exactly were i was going to drill. Without that guide i am sure i would have scratched the hell out of the side of the tank. Slow and easy and lots of water (couple of beer to calm the nerves doesn't hurt either
). When done take the scrap piece off and your off to the races.
Rich