First, arrange the unfrosted cookies onto the platter so you know how many you will need to fill the flag. I was able to make 16 cookies so I did four rows of four cookies. This gave me one blue cookie, seven red cookies, and eight white cookies.
Attach the tip #5 to the bag of frosting with the blue buttercream. Outline and fill in the star using the back & forth piping method.
Place white sprinkles (circles or stars) onto the blue star cookie before it dries.
Remove/rinse/dry the tip #5 from the blue bag and attach it to the red buttercream bag. Outline and fill in all of the stars that will be red.
Remove/rinse/dry the tip #5 from the red frosting and place it onto the white frosting. Outline and fill in all of the stars that will be white.
Allow the cookies to crust/dry for 8-12 hours before arranging on the cookie tray. If you won't be stacking the cookies, but rather laying them flat next to each other, you do not need to wait for the cookies to dry before plating and presenting.