The glass cutter is a clever tool about the size of a pencil. Its han-dle, whether made of wood, metal, or plastic, acts essentially as a holder for the cutter itself, which is either a small wheel or a chip of industrial-grade diamond.
The glass cutter doesn't truly cut glass, at least not in the sense that snips or knives or bolt cutters do. Rather, the glass cutter scores the surface of a piece of glass, much as a utility knife or scribing awl is used to score a piece of wood when marking it for cutting. Given the brittle nature of glass, however, a score on its surface can, given some appropriately applied pressure, result in a neat break along the score line.
Cutting glass takes a little practice. Before you try making any precise final cuts, it's a good idea to experiment a bit on a few pieces of scrap glass. Needless to say, always handle glass with respect: It's sharp, delicate, and inclined to cut its handlers. Wearing gloves is a sensible precaution, too.
To cut a piece of glass, begin by cleaning it. Remove all grit, dust, or other material from both the side to be scored and the underside. Any grease on the surface will interfere with the cutting wheel, so be sure to wipe off any slickness with a clean rag. Use a solvent like paint thinner if necessary.
Next, lay the glass down on a layer of fabric or a few sheets of newspaper. That padding should be, in turn, atop a flat cutting board (a piece of scrap plywood or chipboard on a bench top works nicely). Measure and mark the glass where you wish to cut.
Position a T square or wooden straightedge along the line to be cut. Butt the side of the glass cutter to the straightedge, and pull it gently but firmly along the line. Don't put too much pressure on the glass (especially at the edges where the score line begins and ends be-cause too much pressure can cause the glass to chip). Don't go back for a second stroke: score it only once in a smooth, even motion. You should hear a crackling, grinding sound as you do so. A drop of machine or cutting oil on the cutter will increase the chances of an even cut.
After scoring the glass, put a dowel, wood slat, or perhaps a pencil beneath the glass along the score line. Press down upon the glass, with one palm outspread on each side of the line. The glass should snap neatly along the line.
Any sliver or chips that remain beyond the line can be trimmed off with pliers or with the nibbling slots found on the heads of some glass cutters. Don't be surprised if your first cut isn't perfect. That's why a few practice cuts are a good idea.
In order to make accurate, square cuts, a T square is invaluable. There are purpose-made glazier's T squares available in a variety of lengths. You may wish to purchase one if a good deal of glass cutting looms in your future, or use a simple wood drafting square. For one or two cuts, however, a wooden straightedge (preferably one with a beveled edge) will do.