The icing recipe is below.
I found this recipe at the Wilton site, but we end up adding a lot more milk to make it runnier so you can flood-fill it. The icing we outline with is a little thicker of course.
1 c. powdered Sugar
2 tsp. milk
1 tsp. corn syrup
I also add a lemon extract to it... the recipe doesn't call for it, but I added about 1/2 tsp. per cup of powdered sugar. I also made 3 batches at a time. One doesn't make much!
I use the Wilton paste food coloring.
They take a while to dry (like 6-8 hours)
Start by using a thicker white icing and a number 3 tip to outline each cookie. Thin the icing and add color to it. You can now change to a bigger tip (I suggest #4) to flood fill the cookie. Add candy toppings as decorations while the icing is wet. Then you can go back after the icing is dry and add some thick white outline icing for details. So much fun to make! Get creative!