Pull the front canvas assembly away from the shoulder, and sew the jacket fronts to the back assembly at the shoulders.
The back shoulders are slightly wider than the fronts, so they need to be gently eased in.
Press the seam allowances open.
Then, fold the upper front of the canvas assembly back into place and loosely hand-sew it onto the back shoulder seam allowance.
Sew the jacket fronts to the back assembly along the sides, and press all the seam allowances open.