In the bowl of an electric mixer, beat together egg whites, powdered sugar and cream of tartar for about 3-4 minutes or until mixture is thick enough to hold a peak. You may need to add more powdered sugar, do so in 1/4 cup increments. Humidity/weather can also cause icing to be too loose/runny. You want to be sure you have stiff peaks, so increase powdered sugar until it's stiff. We find 5 cups to do the trick, but adjust accordingly.
Since it dries quickly, keep the icing covered with a damp cloth while working.
Gingerbread House Royal Icing https://pizzazzerie.com/diy/diy-holiday/how-to-make-diy-gingerbread-houses/ December 2, 2019