To assemble the first side of the planter box, lay three planks side by side on a flat surface, then center two vertical support boards along the inside of each mitered edge. Attach these boards to the planks with the nail gun. You can also run a bead of wood glue along the edge before nailing for extra strength.
Once the bottom planks are in place, flip the planter box over (right side up) and lightly sand with a fine-grit sandpaper. It’s best to paint or stain the planter box at this time. If painting, use an exterior paint that includes primer so the wood takes the paint well and is protected from the elements.