Beat the butter in a mixing bowl to soften slightly. Then sift in the icing sugar and mix. If your mixture is to runny, just add more icing sugar to make the icing firmer. Once the buttercream is smooth and well mixed it is ready to use on cakes, or you can store it in the fridge and use later.
If you don't use all of your butter-cream right away you can store in the fridge in an airtight container for several weeks. When ready to use, remove from the fridge and bring up to room temperature.