Mark and cut out two 23 1/2-by-4-inch rectangles from each 29-by-20 inch board, along each 29-inch side. This will give you four 23 1/2-by-4-inch boards and two 29-inch high T-shaped boards.
Make a mark 12 1/2 inches from the 12-inch edge of each T board along each 23 1/2-inch side. Make another mark 4 inches further down on each side, then measure 3/4 inch in from each mark. Cut out the 3/4-by-4-inch rectangles.
Measure in 1 1/2 inches along the 12-inch edge of each T, then 1 1/2-inches from the same corner along a 23 1/2-inch edge. Draw lines toward the inside of the boards from the marks. Where the lines intersect, drill a 7/8-inch hole on each T board.
Draw a line at a 45-degree angle on each T board that connects the first two marks made in Step 3. Cut off the triangle created.
Set the T boards on their tops (the horizontal portions of the "T" shapes) and perpendicular to your work surface, 22 inches apart. Take two of the 23 1/2-by-4 boards and screw them into the 3/4-by-4-inch rectangles you cut into the T boards in Step 2, so that they connect the T boards on each side and all edges are flush.
Set the 22-by-12 board across the 23 1/2-by-4 boards and screw it down to create the seat.
Measure 7 inches down the long edge of each of the remaining 23 1/2-by-4-inch boards, then measure 1 1/2 inches in from the mark and perpendicular to the long edge. Drill a 1-inch hole here.
Measure 4 1/4 inches from each end of each 36-inch edge of the 30-by-36 board, then measure in 5 1/4 inches from those marks and perpendicular to the 36-inch edge.
Attach the boards from Step 1 at the marks from Step 2, perpendicular to the 30-by-36 tabletop and parallel to the 36-inch edge. The 1-inch holes will be closest to the table top.
Place the tabletop assembly on top of the bench assembly and line-up the drilled holes.
Insert the 2 1/2-inch dowels through the holes in the tabletop and use the mallet to pound the dowels into the holes in the bench portion.