For corners of a clapboard building leave a space the thickness of the veneer material. This space is for a vertical corner. Here the vertical "board" on the corner is painted green. The clapboards butt up
against the corner board.   Because the clapboard and corner strip are only an appearance item, this corner does not have to be ninety degrees. There's plenty of room for play.  The foam core is providing the strength of the  building, not the styrene which is only providing the "appearance." The veneer was applied using a thin film of Walthers Goo (contact cement) on the foam core and the
back of the veneer. The corner board can also be glued with this.  Use a toothpick to apply the glue, working from a dab on a piece of scrap.