Here's a bit of advice (love the pun

) from someone who has done this many times.
Size of hole: look at the bulkhead specs, it should tell you the exact size hole. It's that piece of paper that comes in the plastic bag when you buy a bulkhead. Don't have one? Google it...
Find a round hole saw that corresponds to the recommended size. Make a hole on a piece of plywood or pine (I used thin pine, scrap from a produce box/wine box). This will serve as your template.
Securely tape your template to the exact spot where you want to drill the hole. Use lots of duct tape. It will get wet.
Use a dremel with the diamond bit to score the glass, leaning against the template. As soon as you penetrate in one spot, it will be a lot easier to go around, using the side of the bit. LOTS OF WATER is needed here. Squirt gun is a must.
Good luck! It took about 30 minutes per hole, the way I did it.
An aside: After I drilled 2 holes in one tank, I felt so good about myself because of the successful project. Then I set the tank down near the door. Then I went out for a drink. Then came back in, opening the door all the way. Door pushes the new bulkhead, cracking the glass all the way down. Expletives deleted.
Jim