Many people think this is a super hard thing to do but believe it or not, every reef ready tank had to be drilled by someone at some point. It's actually quite simple. I'm going to walk you through it real quick.
I think it should be done with two people, and you'll see why shortly.
First you pick the spot you are going to drill. I purchased this overflow kit from "Glass Holes" and it came with a template, so I just used that to line up my holes-
I could have left the template on as a guide, but Instead I chose to eyeball it. I used a standard cordless drill and the supplied diamond bit that came with the overflow kit-
I got my plumbers putty, found at any home depot or lowes...
I made 2 walls around my 2 hole spots (marked by a sharpie)
I then started with the first hole....I filled the circle I made with the putty with water, and then had my assistant pour water in the hole every time it got low, so to keep the bit and glass cool as I drilled into the glass. I just let the weight of the drill do the work, and I simply steadied it..
And then the other....
Piece of cake.
Don't forget to put something inside the tank like a towel or some cardboard, so when the glass hole falls it doesn't crack the front pane!