See a video tutorial for Step 11 and/or follow the instructions below:
Assembly of the supporting arches. Get the brace spreader (part 11), the reinforcement nogging piece (part 18), and 2 side braces (part 10). Connect the non-notched ends of the side braces using 2 screws. On the other two holes, attach part 11 such that it forms an angle rather than lying on the side braces.
First, connect the notched end of the side brace (part 10) to the junction of parts 4b and 9 by unscrewing the screws. The other end will connect to the junction formed by parts 4h and 2 on the frame. To complete attaching the supporting arch, fix part 11 by running it along with the frame until the screw holes align.