The 2″ x 2″ boards will be the corner supports. Cut the top of the first support at a 5-degree angle with your miter saw, with a length of about 19″ long, and making a parallel 5-degree angle cut along the bottom.
Lay out the planks for a second side and attach them to one of the supports from the first side, remembering to line up the top plank and use spacers below the planks to keep them aligned correctly with the corner supports.