A glass cutter 4.99 at a hardware store and a straight edge. Use the straight edge as a guide and zip the glass cutter down the glass with firm pressure to score it, and immediately place the butt end of the glass cutter ( looks like a ball ) under the cut and with both thumbs press down on both sides of the score. It will snap down the cut you made and then you take a piece of emory cloth ( sand paper) and buff the edges so that they wont cut you. This is the end of Glass Cutting 101
Oh yeah, Glass is very sharp and will cut you. be very carefull.
PS. You only have one try. if it does not break the first time and only a small peice comes off, you will have to try to score it again and then do it from the other end. If that doesn't work, it is ruined and you will have to buy another one.