A jig this size can be a challenge to keep square to the workpiece, so I added a couple of things to help. This drawing shows how the arm fits snugly between two runners in the fence. Plus, a rectangular guide block fits into the slot to help the guide stay square. The router is mounted to one end of the arm (remove the router’s baseplate first). The arm attaches to the fence with a carriage bolt, washer, and threaded knob through the guide block in the center slot.