I've drilled a few. I like 1" fittings. The bulkhead fitting you use will determine the size of the hole you need to drill. I got my fittings and drills from here:
http://www.northcoastpets.com/bulkhead_fittings.htm

I use a bench press so the drill can be stabilized. Doing it by hand is too risky. My press has a movable table and I used wood cribbing to stabilize the tank. I got the rubber boot they sell to hold the coolant. Put some padding inside the tank so when the plug drops out it doesn't break the other side.

Be sure the tank side you drill in non-tempered glass, otherwise it will break. Be sure the tank is securely supported, provide coolant/lube for the bit. Drill slowly using light pressure. It should take at least 5 minutes to bore the hole.

Once it is drilled use carbide or diamond sandpaper to smooth the hole. Use a course grid, a medium grit and a fine grit. Get all the sharp edges. If you skip this step cracks will form and eventually cause the tank to leak.

That is pretty much it.