One way is to make a glass cutter. Make a wood circle the diameter that you want and put a sheetmetal copper or steel edge around it that extends down about 2" or slightly more and put some slits in it about 1/4" x 3/4" or so around it. Mount the wheel on an axle and support over the glass. Power the wheel with a decent motor and lower onto the glass. Feed some 50 or 80 grit Silicon Carbide and water and grind the circular hole in the glass. The process can be further clarified in the ATM books of Ingalls (available at Willmann-Bell - WILLBELL.COM). Before you start, make sure that the glass you have isn't tempered as is will merely shatter after you get a certain distance in. The process isn't fast so expect to take a while.