The first thing you need to do is decide the size of pendant you want. I made mine *just* big enough to fit everything on the back. This is mostly dictated by the size of the battery.
Art glass is a bit softer than window glass. It is much easier to cut as well. Draw your lines on the glass. Dab a little mineral oil on the cutting wheel to keep it moving nice and smoothly. Apply slight and even pressure and roll all the way to the other side of the glass (past your markings).
Glass pliers are very nice and specifically designed to break your glass along your score. They're kind of rounded on the ends and have rubber pads to protect the glass from scratching. The pliers below are regular pliers that have been dipped in liquid rubber several times to get the same effect - mostly anyway. Either way, they will break the glass *way* better than your hand.