Step 4: Cut the metal posts so they will be at least a foot or so underground. I made my bed 1 1/2 feet deep, so the total post length was 2 1/2 feet. Vary the height depending on who will use the bed. Make it slightly taller for gardeners with back problems.
Step 5: Drive the corner posts into the ground with a sledgehammer. I put a 2-inch threaded bolt on top of the posts to create a solid surface for driving them into the heavy clay soil. If you have looser soil, you may want to dig a large hole and pour in concrete to ensure the posts remain in place. Hammer two more posts 4 feet apart on both of the long sides to provide support for the tin.
Step 6: Using 2-inch metal screws, attach the corrugated tin to the outside of the posts, flush with the top of each one. If the metal sheets are not precut, use tin snips to cut them in half. Be sure to wear heavy gloves when working with corrugated tin, because the edges are very sharp.
Step 7: Rip the wood planks (composite or pressure-treated) in half lengthwise with a circular saw so they are 3 inches wide and fit the dimensions of the bed. Clamp the wood strips flush onto the outside edge of the corrugated tin, and screw them to the posts with 2-inch metal screws.
Step 8: Put a wood cap on the raised bed. Cut the wood or composite lumber to equal the length and width of your bed, and then cut the ends at a 45-degree angle. This allows the ends to butt up against one another at a diagonal, forming a strong joint. Screw the wood cap onto the top of the wood strips on the side of the bed using 2-inch deck screws.