These are the instructions for the vegan icing:
In a mixing bowl, cream margarine and shortening until you have a light and creamy mixture with no lumps.
Add powdered sugar and milk and mix at low speed until you have a spreadable consistency.
If you feel that your icing consistency is too thin, add some more powdered sugar to thicken it (1/4 cup at a time).
If you feel that your icing consistency is too thick, add some more milk to thin it out (1 tbsp at a time).