Tempered glass cannot be cut, except with special laser cutters used by professionals. The only way to cut tempered glass is to first anneal the glass, which involves heating the glass to approximately 482 degrees C, and then allowing the glass to cool slowly over approximately eight hours.
Clean your tempered glass thoroughly and dry completely.
Place your tempered glass into a craft oven and set the temperature to 482 degrees C. Once the glass has reached this temperature, turn the oven off and allow the oven and the glass to cool for at least eight hours before removing the glass from the oven. This process is known as annealing, and it removes stress from the glass. The glass is no longer tempered.
Lay the sheet of annealed glass on a clean, flat surface.
Place your T-square or other straight edge along your cut line.
Dip your glass cutter into kerosene for a smoother cut and then grab your cutter as you would a pencil. Starting at the edge of the glass, run your cutter along the straight edge, applying just moderate pressure. Make your scratch with one smooth motion, applying the same moderate pressure along the entire line. Do not run your cutter over the glass a second time.
Place your 1/4-inch dowel under the glass directly under the scratch. Put on your eye protection, and then apply a sharp downward pressure to the glass on either side of the dowel. The glass should snap cleanly along the dowel and scratch.
Rub your whetstone along the freshly cut edge of the glass as if you are lightly sanding the sharp edges. Round the edges slightly. This makes the glass stronger as well as safer to handle.
Always wear eye protection and gloves when cutting tempered glass.
Do not attempt cutting tempered glass without first annealing it. Tempered glass will simply shatter when you try to score it for cutting.