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).
Add a flower shelf, called a false bottom, inside the box. Start by screwing in 2" x 6" crossbeams to the box's support posts (Image 1). With the crossbeams secured, screw in sized 2" x 6" planks to form the shelf (Image 2). Then drill holes through the planks for water drainage (Image 3).
To put the finishing touches on the deck, prepare for staining by scuffing the wood with a hand sander (Image 1). After letting the deck cure over a weekend, add a clear, protective stain with a roller for the flat surfaces, and then use a brush on the smaller areas (Image 2).