Align two of the 4-foot cedar sheets to make a right angled shape. Line up the 2-inch edge of one board with the face of the other board. Use cedar because it is resistant to water damage and rot.
Attach the two 4-foot boards with 4-inch nails. Hammer the nails through the face of one board and into the edge of the other. Hammer the nails until the heads of the nails are flush against the board.
Place the other 3-foot board along opposite edge of the right angled structure. Put the board so that its 2-inch edge is against the face of the other 3-foot board. Make sure that the two boards, whose 2-inch edges are against the face of one board, are across from each other so that the planter box sits level.
Attach the 3-foot boards with 4-inch nails. Hammer the nails through the face of the 3-foot board. Make sure the heads of the nails are flush with the wood.
Align one of the 1-foot by 1-foot sheets with one open end of the planter box structure. Place it so that it lines up with the three sheets of wood and attach it with 4-inch nails.
Repeat Step Four with the remaining sheet of wood to complete the structure of the planter box. Fill the box with 8 to 9 inches of soil and add plants to the planter box.