Tate & Lyle Royal Icing Sugar is a high quality powdered sugar with the characteristic feature that the icing hardens after water has been added. Sift 500g powdered sugar with 75ml of water and stir with a wooden ladle until you have a liquid structure. If the icing is too compact you can add 1 tablespoon of water at a time, add no more than 4 extra tablespoons of water. You can then cover a cake / muffin / cracker / bread with the mixture after which it will cure.
Well-known examples of the use of royal icing are 'Hot Cross Buns', donuts and wedding cakes!