Here's something I threw together real quick at work today to better explain how to get the garage together.
Cut you piece's making sure all edges are clean and line up perfectly.
Then line your pieces up and draw a line on the back side so you know where it sits
Once thats done remove the piece of wood and get your 1/2 square dowel pieces ready to lay out. Using some crazy glue. Run a light line along side of the making you just made for the wall and apply you dowel piece.
What you have left over is simply the edge, just big enough to mount the wall flush with the other piece. Apply a little CA to the dowels side and line up your wall for install.
Add 1/2 dowel piece where you see fit. Some areas on mine needed more than others to make it stiffer and less flimsy. Really hope this helps. Joe