Measure Twice, Cut Once
A solid planter bed starts with good wood. Rough Redwood or Cedar provides the best structure for your planter beds. It's also very important to use untreated wood as the chemicals in treated lumber could find their way into the soil and into your essential edibles. You can purchase the walls of your planter bed in two by 12 foot lengths. For a 12foot planter bed, you will only need to cut the two end pieces. Use your tape measure and mark out two, three foot sections on the two by 12 foot board. Your carpenter's square will provide a straight line all the way around the board when you cut. The four support posts on the inside of your planter bed need to be 11 ¼” high. Measure out these pieces out on your 4x4 post. You'll want to use the carpenter's square here to insure a straight cut all the way around as the saw will not be able to make it through the post on the first pass. And the most important thing – don't forget to wear your safety glasses and keep your fingers away from the blade
When fitting your planter walls around your 4x4 posts, it doesn't matter if you overlap the boards on the end or the side; just make sure you overlap them the exact same way on all four corners to ensure the length of the planter bed is the same on all sides. Grab your bar clamps and place two of them on the corner, holding all the pieces of wood together. If you're feeling finicky, you can use your level to make sure the walls are nice and level.
Full Of Holes
Using a power drill with a 3/8” spade bit, drill two bolt holes on each end of every wall of the planter. Carriage bolts will fit into these holes and keep your planter bed solid for years to come. There are a couple of important things to keep in mind when drilling. Make sure your bolt holes don't intersect on the inside of the 4x4 post or your carriage bolts won't fit. And don't drill all the way through the 4x4 or the opening will splinter and this could also keep the carriage bolts from slipping in easily.
Big Bolt Action
Remove the bar clamps and start pushing the carriage bolts into all the holes around the planter bed. The bolts won't slip in by themselves, so use your hammer and pound them in so they are flush with the boards. Once through, put a washer and nut on the end of every bolt and fasten them securely. You can tighten the nuts by hand, but you'll need a ratchet with a 3/8” head on it to tighten all the way