Unflavored powdered gelatin gives cake decorating gel the supple texture and density needed to create line art and lettering on baked goods. Decorating gel doesn't add flavor, just color and sweetness, so you need only a couple of ingredients to mix up your own batch and adapt it to match any type of cake. Make a large batch of clear decorating gel and divide it into as many portions as you need colors for your project.
Whisk unflavored powdered gelatin into water in a medium saucepan. You need 1 tablespoon of water per envelope of gelatin; one envelope of gelatin makes enough icing for one 12-inch cake when used for line art or writing.
Heat the gelatin and water over medium heat until steam forms. Whisk in 1 cup of light corn syrup for every package of gelatin used.
Heat the gel until steam rises again, then take the saucepan off the burner and stir occasionally as the gel cools. Divide the clear decorating gel into small bowls; use one bowl for each color of decorating gel you want.
Mix 1 drop of liquid food coloring into each bowl of clear decorating gel using a toothpick. Stir until color is evenly distributed, with no streaks. Add more liquid food coloring if necessary, a drop at a time, until you reach your desired shade.
Store unused decorating gel in airtight containers or plastic bags in your refrigerator for up to one month.