Starting with a wall and a gable end, apply the frosting to the bottom and the sides that meet each other, set together at right angles on the foil-covered cardboard base. Brace with kitchen tools (here, a flour bin) and let dry.
Continue with the other two walls. You may have a construction failure or two, but keep re-applying frosting and set the walls back up.
In the lower right side of the picture, you can see that I have attached the fascia (the cookies that decorate the gable ends) to the roof plane and allowed it to dry in place before placing the roof on the walls.
In the upper right of the picture, I have reinforced dried frosting "mortar" with another coat and smoothed the surface.