The problem with drilling holes in glass is that you must keep the glass and the drill covered in water to prevent the glass from overheating. If the glass gets too hot, it will break.
We have invented a reusable acrylic dish-dam-template that holds the diamond drill in place and keeps it covered with coolant as it works its way through the glass. The template is backed with sticky silicone that can be cleaned and re-used indefinitely.
See our illustrated instructions on How To Drill Glass.