Combine the powdered sugar and vanilla. Sprinkle in the milk or water, stirring well with a small spatula and using just enough liquid to form a smooth icing. It should be soft enough to squeeze easily out of a piping bag, but not so runny that it runs out of the bag unbidden. Transfer the icing to the piping bag and decorate the cookies as desired.
Recipe from Homeschoolforms.com, Site Maintained by Donna Young, of DonnaYoung.org