Here's my recipe for perfect Royal Icing to decorate cookies:
1 lb (4 cups) powdered sugar
1/4 t, cream of tartar
3 egg whites
flavoring to taste (usually vanilla or almond extract, but I've done lemon or cinnamon)
GEL food coloring (Michaels has a great selection)
Mix powdered sugar and cream of tartar together in mixer bowl. Stir in egg whites by hand to wet the powdered sugar before turning on mixer. Use whip attachment, and beat first on low speed to blend, then on medium high for 2 minutes. Mixture will be thick, very white and shiny. Add flavoring. I use clear vanilla if wanting any white icing.
If making multiple colors, divide icing into smaller bowls before adding color.
This consistency is good for piping in a pastry bag, but to spread icing evenly over a whole cookie, thin it down with one T. of water at a time, until it is the consistency of syrup, and is "pourable". For the above holly cookies, I add a red M&M while icing is still wet. Let cookies dry on rack. Store any extra icing in fridge, covered with plastic wrap covering surface of icing.