Make and Assemble Flower Box Walls
Lay out three more cut 4 x 4's to serve as support posts for the box (Image 1). Using two of these posts, fashion one wall of the flower box by attaching several 2" x 6" boards (Image 2).
After positioning the wall, continue by screwing an adjacent wallboard spanning to the original support post (Image 3). With the two walls connected, toenail a screw to set the wall post to the deck (Image 4).
Fashion a similar wall for the opposite end of the box, attaching the posts with toenail screws. With the four posts set to the deck, continue to attach wallboards — building up the box (Image 5).