How to Make a Window Box Shelf
- 1). Measure and cut a 1-by-12 inch board to match the length of the flower box the shelf is intended for. Paint or stain the board to match your home's exterior with an outdoor-grade paint or stain.
- 2). Place your shelf brackets against the exterior wall where the shelf will be located, each 2 inches in from the ends of the planned shelf location. Mark the location of the shelf bracket screw holes on the wall.
- 3). Drill a hole at each marked location with a 1/8-inch bit. Place the brackets on the wall to line up with the holes and screw the brackets on with wood screws.
- 4). Place the shelf on top of the brackets and attach it from underneath with 1/2-inch screws fed through the holes on the underside of the bracket.
Source...