Set the cardboard carton on a counter that meets a wall. Spread a thick, even coat of Royal Icing on the back (the side that touched the baking sheet) of one gingerbread end wall and press it onto the carton. Push the gingerbread side of the house up against the wall and use a heavy object to hold the carton in place.
Ice the other end wall. Temporarily remove the heavy object to attach the wall to the carton, then replace it. Let the icing harden for about 15 minutes. Attach the side walls in the same way.
To attach the roof, first trim an even 1/4 inch of cardboard from the perimeter of the taped template so it won't show under the gingerbread. Use a pastry bag to pipe a thick bead of icing on the upper edges of the walls, then gently center the cardboard template on top of them.
Spread icing on both gingerbread roof pieces and attach them to the cardboard roof. Again, use heavy objects, butting them up against the bottom edges of the roof, to hold the pieces in place. Allow the house to set for 30 minutes before decorating it.
