Mix the water, sugar, white corn syrup and cream of tartar in a large cooking pot. It may be difficult to clean off the substance, so use a pot that can be thrown away when the process is completed.
Slowly bring the ingredients to a boil, stirring constantly. Be sure not to heat too rapidly, as this can cause burning and yellowing of the substance.
Insert the candy thermometer and monitor the mixture until it reaches the desired temperature of 300 degrees. (This may take up to an hour.) Once it reaches 300, it's ready to remove from the heat.
Spray the cookie sheet or mold with nonstick cooking spray. Coat it heavily, since the substance is extremely sticky.
Pour the mixture rapidly into the mold. The substance will harden quickly. Spread the mixture evenly. Some bubbles may appear, but they should disappear as the substance dries. Let the substance set for one hour or until it's hard.
Remove the candy glass by heating a knife under hot water, then running the edge of the knife around the edge of the substance in the mold. Alternately, you can heat the bottom of the cookie sheet to loosen the candy glass. Gently remove it from the mold.
Store the sugar glass in the refrigerator until you're ready to use it. If exposed to heat, it will turn sticky and flexible. When you're ready to use it, simply break it as you would regular glass.