Place all egg whites into large bowl. Beat on high with an electric mixer until they form stiff peaks. 

Then add cream of tarter and powdered sugar. Frosting will be stiff.

You want the frosting to be stiff enough to hold your gingerbread house together, but also fluid enough to squeeze through the decorating tube. Some egg are larger/smaller than others, so use your judgment on if you need to add more powdered sugar or not.

Have fun!