In a medium bowl, whisk together 2 cups confectioner's sugar, 1 large egg, and 1 teaspoon pure vanilla extract. Add 1 tablespoon of water one at a time, whisking until the desired consistency is reached. I like to make a batch of lining icing and flood icing, keeping the two in separate piping bags. For lining icing, which is thinner and good for outlining the bones, you want a soft peak to form when a spoon is lifted from the icing. For flood icing, which is looser and used to actually fill in the bones, add more water a little at a time until you reach a consistency similar to maple syrup. When a spoon is lifted from the bowl, the icing should ribbon down, disappearing in 6 to 8 seconds.