I make one large amount of royal icing, then divide it into separate bowls if I'm using more than one color. (Make sure you cover icing tightly with plastic wrap when not using it, as it does dry otherwise!)
For this recipe, that is three bowls of royal icing. Add a few drops of food coloring to each, most easily done with a toothpick.
OUTLINE: The piping icing has to be really thick and needs to hold its shape. Spoon some icing into a piping bag, fitted with a small round tip (I used size 4). You need 3 bags for 3 colors. Twist bag at the end, pushing icing towards the tip. Outline the cookies. Imagine outlining your lips with a lip pencil.
FLOOD: Add a few drops of water to each of the bowls with icing and stir until incorporated. You want the icing to be less thick, easily spreadable, but not very runny. Flood the cookies with this. Using a teaspoon, take some icing and spread it over each cookie. Push it towards the edge with a toothpick. I prefer to work cookie after cookie, so when one is perfectly flooded I move to the next, because icing gets dry and stiff pretty fast, which makes it hard to spread.
When all the cookies are flooded let them dry. If the icing isn't very thinned out, the cookies dry completely in a few hours. If it's very runny, you may need to wait overnight.
DECORATE: Once the cookies are almost completely dry, you can continue decorating, using the very thick royal icing again (first used for outlining). You'll probably already have some left in the before-used piping bags and you don't need much of it. Now trace the lips, also making the line between upper and bottom lip.
Let cookies dry. Once the cookies are dry, you can stack them easily (and eat them, of course)!