Royal icing (recipe adapted from Wilton): Sift 4 cups powdered sugar into the bowl of a stand mixer.
Add 3 tablespoons meringue powder and 5 tablespoons water. Mix until smooth and glossy. Transfer half of the frosting to a piping bag with a #3 tip for piping thin lines and for outlining filled-in shapes.
For flooding/filling in cookie shapes, add more water, ½ teaspoon at a time, to thin the icing to a runny consistency. Add food coloring as desired.