Secure the angle brackets to the ends, sides and the center stretcher. The top of the bracket will be flush with the top of the ends, sides and center stretcher.
Cut the pieces for the seat and lay them on a work table or other flat surface. Position the bench frame upside down on the seat pieces (the seat pieces will overhang by 1/4" on the sides and ends) and secure using 3/4" screws through the angle brackets. See the drawings below.