Cut the planks to size and drill 1 1/2" pocket holes on one edge and both sides of each board. Attach them together with 2 1/2" pocket hole screws.
Cut the headboard frame pieces to size and drill 1 1/2" pocket holes on one edge and both ends of the horizontal boards. Attach the side frame pieces to the top frame piece with 2 1/2" pocket hole screws and wood glue.
Place the planked assembly inside the headboard frame and attach it to the frame posts with 2 1/2" pocket hole screws and wood glue. Follow up by attaching the remaining vertical board.
Cut the back stretcher to size, drill 1 1/2" pocket holes at both ends and attach it between the vertical posts, 6" from the bottom, with 2 1/2" pocket hole screws and wood glue.
Cut the top trim piece to size. Center it over the top of the headboard frame and attach it to the headboard with 2 1/2" pocket hole screws and wood glue, through the pocket holes in the top frame board.