Lay the swing on its back. Align the 1"x4" piece with the top of the backrest and nail in place.
Place (1) 1"x6" board on the top of the arm rests. Align and check for even spacing from the top trim. Nail in place.
Place the remaining 1"x6" board directly underneath the arm rest. Align and check for equal spacing from the seat frame. Nail in place.