Parts:

(1) Seat unit
(1) 3/16" rope (4 ft.)
(1) Clam cleat with

 

hardware
(2) 2" x 8" self-adhesive hook tape

Find the appropriate seating position. Mark where seat sits and stick hook tape to the hull. If hole for backrest rope is not already drilled*, drill with ¼" drill and knot one end of rope and pull through, running it through the back of the seat and into the clam cleat.
Install cleat onto the underside of the deck, just under the cockpit rim, within easy reach. Use 3/16" holes. Run rope into cleat. (Cleat will only jam the rope properly one way, so if installed in the wrong direction it won't work.)

*Builders of wooden boats may want to mount a wooden block with 1/4" hole drilled in it to the underside of the deck on the opposite side to the cleat to accept the knotted end of the rope.