Score the stained-glass sheet along where you wish to cut it using a scorer specific to glass cutting. Use a strong steel ruler if you want the break to be straight. Be sure to press hard enough with the scorer so that you actually see the line, otherwise the score may not be deep enough and the glass will not break in the correct place. The score line also needs to run from one edge of the glass to the other rather than stopping in the middle of the sheet, this helps to keep the cuts controlled and not waste glass.
Break the glass along the score line. As stained glass tends to be bought in large sheets, mosaic artists will need to be able to cut both large and small pieces of glass down to size. For large pieces this can be done by hand; carefully pressing the edge of the glass on a table until it snaps. Small pieces are best broken using a pair of running pliers as the pliers can grip the smaller area of glass better than fingers and cut it neatly.
Tap the score lines of curved cuts with the steel ball on the end of the glass scorer before breaking. By firmly tapping a curved score line it cracks the glass beneath the score and helps the glass to break along the curve. Be aware that when cutting smaller pieces of glass like this, the glass can suddenly break along the score line before expected.