Prepare the Dough: Make the sugar cookie dough according to your recipe. Chill it until firm enough to roll out without sticking or losing its shape.
1 batch cut-out sugar cookie dough
Cut the Cookies: Roll out the chilled dough on a lightly floured surface to about 1/4-inch thick. Cut out star shapes using a 1 1/2-inch cookie cutter.
Bake the Cookies: Place the cookies on a parchment-lined baking sheet and bake according to your sugar cookie recipe, usually until the edges are just set. Let the cookies cool completely before decorating.
Make the Icing: Prepare the royal icing according to your recipe. Divide it into three portions, about 1 cup each if decorating 30 small cookies. Leave one portion white, tint one red, and tint one blue.
1 batch royal icing, Red gel food coloring, Blue gel food coloring
Fill the Bags: Fit three decorating bags with couplers and size 3 or 4 tips. Fill each bag with one icing color and secure the tops with rubber bands.
Set Up the Sprinkles: Pour the red, white, and blue sprinkles into three separate shallow bowls so they are ready to use as soon as each cookie is iced.
About 1/3 cup red sprinkles, About 1/3 cup white sprinkles, About 1/3 cup blue sprinkles
Ice the Cookies: Pipe an outline around one cookie, then immediately fill it in with icing using a back-and-forth motion. While the icing is still wet, gently press the iced side of the cookie into the matching color of sprinkles until covered.
Dry the Cookies: Place the decorated cookies on a flat surface and let them dry uncovered overnight. Once fully dry, they can be stacked or packaged.