Stand a 2-by-4-inch board that is 6 feet long on one of its 2-inch sides. It should stand 4 inches tall. Place a 4-by-4-inch piece of lumber that is 8 inches long on its end at one end of the longer board. This smaller piece of lumber is a corner piece of the planter box.
Line up the flat end of the longer board with one long edge of the smaller piece of wood. The adjacent side of the smaller lumber will be flush with the longer board's end. The bottom of the smaller lumber will be flush with the long, flat edge of the longer board. Nail the boards together in this position.
Repeat Steps 1 and 2 to nail another corner piece on the other end of the longer board. The parts of the corner pieces that jut past the longer board should be on the same side of the longer board. Repeat the steps again for both ends of another 2-by-4-inch board that is 6 feet long. Now you have two long boards with corner pieces nailed to each end. They should stand up on their own.
Place the two longer boards parallel to each other 2 feet apart. They form the two longer sides of the planter box. The corner pieces should be inside the box.
Place a 2-by-4-inch board that is 2 feet long on one 2-inch edge so that each end covers a flat edge of one of the longer boards and the adjacent sides of the corner pieces. This shorter board should complete one side of the box with the two corner pieces inside it. Nail this board it into place on both corner pieces.
Repeat Step 5 for the other end of the planter box. Now you have a four-sided planter box that is 4 inches tall, except for the corner pieces that are 8 inches tall.
Stack another board on edge on top of each side board. Nail them into place on the corner pieces. Now you have a box that is 8 inches tall all the way around. Each side consists of two boards standing on end.