Measure out the length and width of the frame with a tape measure, and make a cut line over four 2-by-6-inch pressure-treated frame boards. Two boards will form the sides of the frame and two will form the length. Use a circular saw to cut frame boards.
Measure support boards to go across the interior length of the frame every foot. The supports will be 4 inches shorter than the side frames. Cut the support boards with the saw.
Sand the boards with sandpaper or a sander to smooth the surfaces.
Arrange the support boards across the leveled site so they are spaced 1 foot apart. Place the frame boards around the outsides of the supports.
Assemble the frame by having a helper hold one of the back length boards on its side. Hold a side frame board against the end to form a corner and attach each board with a drill and four 2 ½-inch galvanized screws.
Place a support board on its side 1 foot from the end of the frame and attach it to the length board. Continue attaching the support boards to the length board every foot until you attach the other side of the frame.
Hold the final frame board parallel to the back frame along the exposed ends of the supports and side boards. Attach the front frame to the supports by drilling 2 ½-inch screws through the frame into each support board, using four screws on every board.