So, starting with the gelatin.... We've been using plain, unsweetened, unflavored gelatin from the supermarket. There is a commercially prepared (vegetarian) version of a gelatin printing tablet available called a "gelli" tablet, but I have not investigated this. Yet.
After finding a squarish or rectangular pan, pour in enough water to create a 1/2" deep pool. Transfer the water to a measuring cup before pouring it into a sauce pan. You will want to know the amount of water you're using so that you can add 2 packets (2 tablespoons) of gelatin for each cup of water. Bring the mixture to a boil, stirring frequently. Remove from heat and let cool. Line your squarish pan with plastic wrap for easy removal, and pour the cooled mixture into the pan. try to poke out the air bubbles on the surface, unless you think you'd like to have them in your prints.
Allow the gelatin to set over night in the refrigerator. Remove the whole tablet and place it on a flat pan. We used an old baking tray.