Place the butter in a mixing bowl and mix for about 5 minutes until light and doubled in quantity. Some people just add the butter and sugar then mix but mixing ur butter first makes a big difference. It will make the butter very airy and pale which is good if you are not adding any coloring so it doesnt have a yellow color.

Start adding the sifted axing sugar a cup at a time and mix until well incorporated before adding another cup.

Once all the icing has been added, start adding the milk one tablespoon at a time until you get the desired consistency. If you are going to pipe it, you want a lighter consistency but if you will use the butter icing for spreading then you need a thicker consistency so you wont add as much milk. This is also the time to add the flavor and color until you get the color you need. You have to be careful while adding color because a little goes a long way. Add a drop at a time and mix.