Measure and cut all the board lengths: 2 two-foot boards and 2 one-foot boards.
Place the boards on edge with the two longer boards in the front and back and a shorter board at each inside end. This should look like a frame.
Use two brackets at each inside corner to attach all four boards together. A bracket should fit 2 inches from the top edge and the other bracket 2 inches from the bottom edge at each inside corner. Screw them in with 3/4-inch screws, the drill and screwdriver attachment.
Place the planter box frame on top of a scrap piece of plywood that is at least 2 feet long and 16 inches wide. Draw around the outside of the wood frame onto the plywood.
Cut the plywood along the pencil markings. Place the cut plywood on top of the frame. Use screws every 3 to 4 inches at the outer parameter of the plywood to attach it to the frame edges.
Attach a 1/2-inch round drill bit to your drill. Drill holes through the plywood, randomly. These holes will allow extra water to drain. Water sitting at root ends can cause root rot.