With an electric mixer on low speed, beat ingredients until fluffy, 7 to 8 minutes. Use immediately, or transfer to an airtight container (royal icing hardens quickly when exposed to air) and refrigerate up to 1 week. Stir well with a flexible spatula before using.
Thin icing as needed by stirring in additional water, one teaspoon at a time. For piping designs, add just enough water that icing is no longer stiff; for floodwork, add water until icing is the consistency of honey.
To prevent icing from hardening, cover bowl or piping tip with a damp paper towel, and wrap in plastic.
