Build the planter walls out of untreated 2-by-12 lumber. Cut the lumber into four pieces that are each 6 feet long. Paint the boards with latex outdoor paint and let them dry completely.
Attach two of the boards at their ends using a metal right angle joint bracket secured with screws to form a perfect angle (or L-shape). Do the same with the remaining two boards so that you have two identical pieces.
Put the two angled pieces together to form a perfect square. Secure the two remaining corners with right angle brackets. Your planter box walls are finished; place them around the section of ground where you wish to create a garden plot.