Place sugar and cream of tartar in a mixing bowl.
Gradually add egg whites, whisking constantly with an electric mixer until the mixture forms a thick white paste. The whisk should be able to stand upright in the mixture.
Add more icing sugar if necessary.
Keep icing covered in an airtight container or covered with a damp cloth at all times to prevent hardening. To use, place in a zip-lock bag and push the icing down to one corner. Snip a tiny corner from the bag and pipe icing.
When making Royal Icing it is important to use pure icing sugar to create a stiff icing. Do not use soft icing mixture as this contains cornflour. The cream of tartar ensures a pure white icing. Royal Icing dries hard and fast making it ideal for producing intricate piping work and is wonderful as a glue for constructing gingerbread houses.