ART OF STAINED GLASS
STEP 1: Score the glass with a cork-backed, steel ruler if necessary. Put enough pressure on the ruler along the length of the score to avoid slipping and shifting of the ruler.
STEP 2: Hold the glass as shown, one hand on each side of the score line. One side of the glass which is perpendicular to the score should lay on the table. The other side of the glass which is perpendicular to the score should be elevated, and is the side that the hands hold. Place the thumbs parallel to the score, above the glass, and the other fingers curled under the thumbs, beneath the glass.
STEP 3: Put pressure with your hands to force a downward motion of the glass on both sides of the score line, much like closing a book where the score line would be the spine, and the glass sheets to separate, the covers. Depending on the type of glass used, be prepared to apply considerable force with your hands. A shorter score will require much less force than a long one.
page 7 (how to cut glass, by manufacturer)