If using piping tips, place inside piping bags (pastry bags). Partially fill piping bags with desired colors.
Start by piping a general outline using a #2 piping tip. (#0, #1, #3 also work) By the time you have outlined all your cookies, the first outline should be dry and ready for flooding.
Thin some royal icing using warm water. Add ½ teaspoon of water at a time to your royal icing until it is loose and pourable. Using a squeeze bottle, piping bag, or spoon, flood the inside of the outlines with the thinned out royal icing. The icing should naturally flow to the edges of the outline. If it needs a little help, you can use a clean toothpick to drag it outwards. Use a toothpick to also pop any air bubbles that may have formed on the surface. Let the royal icing dry at room temperature. This takes about 15-20 minutes (something more depending on the humidity of your room).
Once dry, use the piping bag used for creating the outlines to pipe additional decor on top of cookie to enhance shape and texture.
Allow to dry completely before stacking. Store at room temperature. Keeps for 3 days.