Step 1, mark your hole
Step 2: Use plumbers putty to make a dam around the hole and fill that with water
Step 3: Use some tape and tape the back of where your hole will be, because the last thing you want is for the weight of the glass to fall and chip the tank from the inside.
Step 4: Start drilling, go slow at first to make a groove so your drill doesn't slide all around (yeah it will), then once you get your groove, go medium speed, but don't put much pressure, diamond drills work by grinding away the glass, not like wood drills which carve into it. Periodically refill your water as it splashes all over the place. And make sure you drill perfectly perpendicular to the glass, I know the temptation would be to lean and rock it from side to side to "bite in" a bit more, but again glass drilling doesn't work like acrylic/wood.
Eventually, done. Remove your tape + glass slug, and clean up. Don't rub your finger around the hole though, you might get a nasty surprise like I did (yes cut glass is strangely really sharp

)
But yeah, glass bits, ebay/amazon, wait a few weeks for them to get here from hong kong or china. If you borrow one bbe kind enough to pay the guy you borrow it from a fraction of the cost of the drill or buy him some lunch or something, as the diamond dust will come off.
