Decorate horizontally, not vertically

Some people also decorate their pieces before assembly (refer to Teresa Layman's book, "Gingerbread for All Seasons").

For example, if you are trying to create New England style clapboards in royal icing, it is much easier to pipe the siding onto your walls when they are horizontal, vs. trying to pipe straight siding onto a vertical wall.

After your walls are decorated and completely dry continue with your assembly as normal.