Tempering glass can be quite the challenge. It requires heating glass to a very high temperature and then allowing the glass to cool quickly. This alters the molecular composition of the glass in such a way that the glass becomes up to 6 times harder than normal glass, but at the same time it is more brittle. One advantage to tempered glass (besides the extra strength) is that if tempered glass should break it crumbles into safe chunks rather than sharp and potentially dangerous shards.
Shape or drill any holes in your glass before tempering--once your glass is tempered it cannot be cut or drilled without shattering into small thumbnail-sized pieces.
Heat the glass you wish to temper to a temperature of at least 600 degrees Fahrenheit in an oven or a kiln.
Remove the glass from the oven or kiln (carefully, using tongs or a special paddle made for handling hot glass) and set it on a brick or cement surface and allow it to cool quickly by fanning it with cool (but not cold) air. It is the quick colling of the outside surface of the glass that causes the molecular changes that make the glass tempered. Allow the glass to cool completely.
Check to see if your glass has been properly tempered by looking at it with polarized glasses and shinning light through it. Tempered glass will have patterns of shadows (sometimes called "quench marks") visible with polarized lenses.
Tempered glass occasionally shatters spontaneously due to the internal stress that the tempering process creates.
Use care when handling tempered glass as it scratches mush more easily than normal glass. Do not drop or strike tempered glass. Always wear safety goggles and heavy gloves when working with hot glass.