Choose your screw locations, depending on how high you want the seat and how far back you want the back. Drill pilot holes in the sides, seat, and seatback. (Before assembling, I used a hand router with a roundover bit on all of the edges of each of the pieces.) Screw the seat and seat back in place.
Optionally, you can countersink the screws and then cover the holes with wooden buttons. Or, you could choose your own method of attaching the seat and seatback. I’m not the boss of you.