Make the frame for the planter. Nail two 26-by-1-by-2-inch boards to two 13-by-1-by-2-inch boards to form a rectangle. The shorter boards should be on the outside of the longer boards, on the top and bottom, so that the rectangle will form a 26-by-16-inch-wide frame. To make sure that the frame is extra secured, you can use wood glue to hold the frame together before nailing it. Repeat this process to make two separate frames.
Place the frames on their sides and apply a strand of wood glue in the inside face of the long boards. Then, attach four of the 5 ½-by-12 ¼-inch boards to each side by nailing them into the long boards from the inside out. Attach another four 5 ½-by-12 ¼-inch boards to the opposite side of the frame. The box should now look half complete.
Stand the box on its end. Apply another strand of wood glue to the inside face of the bottom end of the frame. Attach two of the 5 ½-by-12 ¼-inch boards to the end of the frame and repeat the process on the other end of the frame. You may need to nail slightly up and at an angle to keep the nails from exiting the exterior of the frame.
Glue the two 1-by-6-by-24-inch panels at the one end of the frame. This will form the base of the planter. You should then drill several holes into these panels so that water can leave the planter easily.