I did want to add another recipe that I have that is what I use for cookies. It is like royal icing in that it dries up hard but it is also shiny, smooth and it is soft when you bite into it.
Mix 1 cup powdered sugar and 4 tsp milk in mixer, stir until very well mixed.
Add 1 tsp almond or clear vanilla extract and 1 tbsp of light corn syrup.
This recipe for icing is much less fragile than royal icing. It is a very wet icing and drips a bit but you can add more powdered sugar if you want it a bit more stiff.
What I usually do is spread the icing in one color for kind of a background color. Then i let it dry (usually takes about an hour or two). Then what I do is pipe on more details on top of the cookies.
You could also outline the cookie first in the designs that you want in a stiffer consistancy of this recipe and then put the more wet icing in a piping bag and fill in the gaps after the outlines dry.