Stand two leg boards (24 inches) upright and parallel to each other. Arrange them so that their wider sides face each other and then place a leg top support board (14 inches) on them. Arrange the leg boards in the center of the top support board and then slide them out to the edges of the board so that the outer side of each leg board is flush with the end of the support board.
Drill two holes down through the top support board and into each leg board. Attach the boards with 2 1/2-inch wood screws driven through the drilled holes. Repeat for the other set of legs.
Arrange four seat boards (48 inches) parallel and touching each other with their wide, long surfaces touching the working surface. Position the assembled legs upside down on top of the seat boards, with each assembly about 6 inches from the end of the seat boards. Drill two holes through each leg support board and into each seat board, for a total of 16 holes, being careful not to break through the top surface of the seat boards. Attach the boards with 2 1/2-inch wood screws.
Hold side leg support boards (also 14 inches) in place along the outside edges of the leg boards, halfway between the seat and the floor. Drill two holes through each leg and into the leg support board. Attach the boards with 2 1/2-inch wood screws.
Turn the assembled seat upright and hold the board for the back of the bench (48 inches square) in position against the seat. Drill evenly spaced holes every 12 inches through the rear board and into the seat boards, the leg boards and the leg support boards. Attach the bench back with 2 1/2 inch wood screws.