Step 3: Seat and back
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.