First make sure your aquarium isn't tempered glass, if it is, it will shatter when you attempt to drill it.
Second, you need to aquire a diamond glass drill bit. They can be ordered online or sometimes you local glass shop may already own one of the dimensions you need, in which case you may be able to pay them to drill the hole for you.
And finally, you need a varispeed drill.
Personally I found it best to use a hole template, squirt it regularly with windex while drill and just drill with nice steady pressure. If you push too hard at the end, it will break out and leave the edges rough.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).