The answers given so far are on the right track. If you are using a glass cutter wheel, you need to build a balance apparatus to ensure that the scoreline is completely even. The method of heating and cooling is the scoreline is probably the safest--tie a piece of metho-soaked cloth around the scoreline and set it alight, then run a fresh icecube around it immediately the flame falls off. The more traditional method of gently tapping the glass on the inside of the scoreline will not give you the straightest/safest cut. In any case you will probably have to put protective stripping over the cut, or take it to a glazier for grinding--in which case, I would let the glazier grind the whole thing in the first place--his risk, not yours. PS: Why not experiment with a plastic one first? You may be saving some land-fill.
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Get a glass cutter, score around the top edge and then heat the glass around the score. When the glass is a little hot dunk it in cold water and repeat the heating and dunking until it cracks.
On smaller objects I've also seen folks run an ice cube along the heated score line to induce the stress needed to create the crack.