Step 3: Seat and back
There should be eight 14" pieces left from the long side sections of the crutches. Sandwich those around the oak pieces to make a slatted seat as shown. Pin together with one screw from the top and two from the bottom. The seat has four slats; the back has three.
Space the seat slats about an inch or two apart, with the end slats flush with the ends of the crutch pieces as shown.
For the back, use the top sections of the crutch sides, which should have pegs in the ends for interfacing with the armpit supports. Space the slats the same as the seat, but leave the fourth slat off at the pegged ends. Screw the same as the seat.
If you are not planning to disassemble, sand, and finish, then reassemble, glue as well as screw the pieces together.