4 cups confectioner's sugar
3 large egg whites
food coloring (optional)
Mix ingredients with electric mixer until smooth. It helps to add a bit less confectioner's sugar than called for initially. You can then add small amounts until the icing is thick enough. Depending on the size of the eggs, this may be more than 4 cups.
For attaching decorations, the icing should easily squeeze through a small pastry tip, but shouldn't be so thin that it drips. For assembly, use a very large pastry tip and make the icing very thick. Use pastry bag or butterknife to apply as desired. Color with food coloring if desired.
Keep unused portion covered with a damp towel to prevent drying. It will dry out pretty quickly, so do this even while you're working with the icing. If you need to keep the icing overnight, cover the bowl with the towel with plastic wrap and put it in the fridge.