One of the myriad ways to use Con-Tact paper is to to coat the back of your stained glass pattern before cutting it. The Con-Tact paper gives your pattern extra strength to withstand multiple use. If you're one of those people who grind glass with the pattern attached, this helps the pattern survive, to some degree.
Con-Tact paper can also be cut into shapes and used as a stencil, with paint on any flat surface, or with Armour Etching Cream in order to etch clear glass.
Another use is place it, sticky side up, at the bottom of your coaster mold, and then set your mosaic pieces on it. The Con-Tact paper will hold your pieces in place while you pour your cement over them. (This is a theoretical use. We tried it with stepping stone molds and it didn't work so well. Your mileage may vary.)