The proper consistency is when the ribbon of icing that falls back into the bowl remains on the surface for about 5 seconds before disappearing. Another way is to take a cookie and place a small amount of icing in the middle of the cookie. Using a small knife or spatula, push the icing to the edge of the cookie. If the icing runs off the edge, thicken the icing by adding a little more confectioners sugar. Do not add too much sugar at once.