it’s easy! Use a fork to mix 1 cup of powdered sugar with 4tsps of milk. Customize the consistency by adding milk to make it thinner, or more powdered sugar to make it thicker. The reason cookie icing hardens is because it is low in fat, so do not add butter or use a heavy cream. Use a plastic sandwich bag with a hole cut in one corner to pipe the icing onto the cookies, or, with these cookies, you can just plop it on there with a spoon if you want. ***The store-bought Betty Crocker cookie icing is less transparent and blends in better with the marshmallows. I think it looks better than homemade.***