Step 1: Outline just inside the edges of the cookie—use royal icing in a piping bag fitted with a small round tip. Let the icing set slightly, 20 to 30 minutes.
Step 2: Fill in—or “flood”—the center of the cookie with more icing. (If you’re going for an ultrasleek look, thin the icing with a few drops of water.)
Step 3: To fill in tight corners, use a toothpick to drag the icing outward. You can also use the toothpick to pop any air bubbles that form. Let the icing dry partially before adding sprinkles, and completely (four-plus hours) before piping other colors on top.