The first step in decorating is to apply the icing, which involves piping the border with a piping bag, then filling in the center. Yee recommends making two consistencies of royal icing, one for each step. “You want a firm icing for the border, and a looser one to fill or ’flood’ in the border, which can be done by adding a touch of water to your ’flooding’ icing,” she says. You can use a piping bag, an offset spatula, or a paring knife to frost the center, and toothpicks can help to make designs, spread icing into detailed corners, and pick up mistakes.
Quickly add the sprinkles while the icing is still wet and tacky—within two minutes of frosting. Though the surface of the icing will feel dry after about 10 minutes, it’s important to let it fully harden for about four hours. And don’t stress too much about achieving perfection, she says: “Be patient and have fun! They’re cookies, so do yourself a favor and don’t take the icing too seriously.”