Butter icing is basically butter and icing sugar, but you can also add milk, sugar syrup, fruit juice or cream. You should come up with an icing that is smooth, spreadable and firm enough for decoration. If the end result is too soft for piping, you can either reduce the quantity of butter or beat in more icing sugar.
To cover and decorate a cake with icing, you must first spread a thin layer of icing sugar all over the cake. Spoon about one-third of the icing on top and spread out evenly. Then, using a ruler or palette knife longer then the width of the cake, draw the icing at an angle of 30° across the top of the cake without applying pressure.

Trim the edges and remove surplus. Spoon a further third around sides evenly. Then, using an icing scraper or palette knife, smooth and flatten the sides. It will be easier if you have a rotating plate. Hold the palette knife upright at an angle of 450 on the side of the cake and, as you draw it towards you, rotate the plate with your other hand.

Here is a recipe on creamy white frosting.

More low carb recipe.