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Measure the window opening. A window box should be the same width or slightly larger than the window.
Cut the two long side pieces and one long bottom piece from 1" x 8" pressure-treated boards to the length determined in Step 1. Cut two square pieces from a 1" x 8" board to form the ends of the box. Rout the top edges of the box pieces for a finished look.
Glue, clamp and screw the window box together using 3" galvanized screws. Stain or paint the box before installation if desired. Drill four 1/2" drainage holes along the length of the bottom piece.
Fabricate or purchase 45-degree window box brackets and mount them to the house using 3" galvanized screws. Use a level to make sure they are even. Position the window box on top of the brackets and attach to the house by screwing through the back panel with 3" galvanized screws.