To create the support for the front facade of your miniature dollhouse scale cabin, trim then glue a ten inch piece of ½ inch square basswood stock to each side of the back of the facade so that it lines up with the outside wall edge and the bottom edge of the facade. Make sure the side support does not stick up above the roof line. It should just match the top of the side wall. Clamp it to the front to ensure the facade is thoroughly glued to the side support. Trim a piece of 3/16 square basswood stock to fit between the two side supports and glue this in place between the two side supports.
When the glue has dried drill a small pilot guide hole in the side support where it lines up with the cross support. Use wood screws to join the cross support to the side support. If your shelf protrudes behind the side and cross supports (in order to put a scene into your window later), mount a scrap piece of 3/16 square stock to the cross bar in the centre of the display, to make the cross bar match the distance from the front facade to the back of the shelf. This will be the site where the wire or picture hook will be located to hang the shelf.
Cover the sides with 1/16 inch thick basswood siding stock as you did the front. Finish applying the batten trim to the sides, avoiding adding a top piece on the side wall (it will interfere with the roof).