Step 3: Marking the stock for the locking dowel
First thing to do is get your scrap wood for the stock pretty square and flat. There are several ways to do this. Because the piece I had was small, I used a jack plane to get everything set. It probably took me about a minute. Doesn't have to be perfect. If you use power tools to face the stock, then please use appropriate clamps and push sticks.
First, let's mark the line that will define where our locking dowel will go. Use your combination square and a pencil to scribe a line across the face, slightly more toward the "top" of the stock than to the bottom. Use your combination square to transfer the line across all faces and edges.