Stand up all four legs, and position the leg support board on top of them. Move one leg to each corner of the support board, so that the outer two edges of the table legs line up perfectly with the outer two edges of that corner of the support board. Predrill three holes into each leg, passing through the support board and into the table leg. Secure the legs in place using the 3-inch wood screws.
Hold one of the side boards in place between the table legs on the smaller side, with the wider surface facing outward and the thinnest surface flush with the support board. Position the side board either flush with the outside edges of the legs or inset 1 inch into the center of the table. Predrill three evenly spaced holes down through the support board and into the side board. Attach the side board to the support board with the 3-inch wood screws.
Predrill a hole into the bottom surface of the side board, 1 inch from each end and angled at a diagonal into the adjoining leg. Secure the boards together with a 3-inch wood screw, sinking the screw into the wood until the head is not visible from the side. Repeat these steps for the other side board and the front and rear boards, keeping them uniform and either all set flush or all inset 1 inch between the legs.
Flip the assembled table upside down and set it on top of the underside of the tabletop. Position the assembled legs evenly in the middle of the tabletop and predrill eight evenly spaced holes through the support board and into the tabletop, being careful not to break through the top surface of the tabletop. Attach the support board to the tabletop with 1-inch wood screws.