How to Cut Glass

cutting isn't as hard as it might seem, as long as you have the right instructions. Nobody is perfect with glass, but you can get it right nearly every time. Here's how to cut 1/8" (3.1 mm) clear glass, also known as "double strength" (DS), which is probably the easiest to cut, as well as the most common..

This method has dimensional limitations. Cut cannot be very long, e.g. 24 inches is long, and failure rate is too high. Also, the pieces to be cut have to be wide enough, minimum some 6 inches perpendicular to the cut.
 

Steps
Clean the surface, but only along where you plan to score. Any grime or silica (the substance that makes two pieces of glass slide on top of each other to prevent scratching) will ruin your score and make you think you cannot cut properly. Just run your finger along the surface of the glass where you plan to score.
Obtain a glass cutter and some light oil. You can buy cutting oil at a stained glass store or use a small amount of kerosene. Dip the cutter in the oil before you score the glass. An oiled cutter creates a smoother score line.
Grasp your cutter like a pencil. Make sure to look at the wheel and its orientation to the glass and align it properly. This is a very simple grasp to achieve, so do not unduly complicate it.
Use a scrap piece of glass to practice on. Apply pressure to the glass as you pull the cutter along the surface, rolling on the small carbide wheel. The score must run from one edge of the glass to the other edge. Listen for a smooth sound like ripping silk. A gritty sound means that you are pushing too hard or that you did not oil your cutter. The less sound you make, the better your score will be. If you push too hard (a very common mistake), your cut gets "hot", meaning that it snaps and pops. Try it and you'll see. You are aiming to get a uniform score. If you are pressing too hard in one spot and perfectly in another, the glass will not break the way that you want it to. Microscopic imperfections in your score will cause your cut to go askew.
Score along a straight edge. Use a yardstick (meter ruler) or a normal desk ruler. Use something that has a high enough profile so that it won't conflict with the wheel on the cutter, as would a ruler with a thin metal edge.