Cut 3/4-inch-thick cypress or cedar boards to following dimensions:
(3) 8" wide x window length
(2) 1-1/4" wide x window length
(3) 1-1/4" wide x 5-3/4" long
(2) 8" wide x 11" long
Cut 2x4 pressure-treated lumber into three (3) 8" long pieces.
When it's full of plants and soil, a window box can be very heavy. The best way to mount it is to properly secure it to the exterior wall using cleats. To create cleats, cut three 8" pieces of pressure-treated 2x4. Hold the 2x4 tightly against the exterior wall and drill two pilot holes where you'd like screws (Image 2). Drive two 3/4-inch concrete screws through pilot holes to secure cleat to wall (Image 3). Repeat process on other cleats.
Place bottom of window box flat on work surface. Stand front of box on end and press tight against side of box bottom. Clamp or hold in place, then drill pilot holes every eight inches using 1/8" drill bit (Image 1). Insert 1-1/4" screws into pilot holes (Image 2). Remove clamps (if used) and repeat on back side of box.
Cut two 1-1/4"-wide trim pieces to exact length of front panel of window box. Make sure that each trim piece is flush with top and bottom of front panel, then tap into place with nails. Position three 1-1/4" wide x 5-2/4" long pieces on left and right side of box and one centered. Tack into place with nails.
Using a sash brush, apply primer to window box. If using a dark color like red or black, start with a tinted primer coat for better coverage. Allow primer to dry, then apply one to two coats of exterior semi-gloss paint.
For a smooth finish on the window box, fill nail and screw holes with wood filler before painting. Allow to dry, then sand entire box with medium-grit sandpaper. As this is an exterior project, this finishing step is optional.
Holding window box in place against cleats, drive four screws through back panel into each 2x4 block.
If you wish, decorative brackets can be added under the window box for additional support and architectural detail.
Add a 1- to 2-inch layer of gravel in the bottom of window box, to allow excess water to drain. Cover gravel with potting soil, filling box to about an inch from top. Add plants and thoroughly water.