"Flooding" is a technique used to cover a cookie completely with royal icing. Here's how to do it: Transfer about 1/2 cup of the royal icing to a resealable plastic bag and snip a small corner. Use this icing to pipe a thin border around the edge of each cookie. Let set for a few minutes. Thin the remaining icing with a few drops of water until it's the consistency of syrup. Place half of the thinned icing in a resealable plastic bag and tint the rest with red and/or green gel food coloring. Transfer the colored icing to bags. Snip a corner of the white icing bag and pipe a generous amount inside the border of each cookie. Using a small offset spatula or the back of a spoon, spread the icing to cover the cookie. Decorate with the colored icing while the white icing is still wet. Let the cookies set at room temperature, at least 1 hour.