Screw together two 47.5-inch-long boards to two 16-inch-long boards in the shape of a rectangle using four 2-inch-long screws for each corner. This is your seat box.
Screw six of the 16-inch-long boards into the seat box to serve as seat supports. Space them every 4 inches, and attach them using the 2-inch-long screws.
Arrange four of the 47.5-inch-long boards over the seat supports, spacing them evenly. Screw each board in place using eight 0.25-inch-long screws per board, only fastening to the four innermost seat supports.
Flip the seat over and place the 16.375-inch-long posts in each corner of the seat box. Hold the posts firmly in place and secure them using four 2-inch-long screws per corner.
Attach the remaining long and short boards to the bench's legs about 4 inches below the seat box to serve as support beams. Screw the boards onto the outsides of the leg posts with 2-inch-long screws, using four screws per corner.
Sand the edges and corners of the bench. Paint or stain the bench to match the decor of your mudroom.