Measure the width and the depth of the window ledge.
Cut two pieces of lumber to the width of the window ledge for the front of the box and one for the back using a circular saw.
Cut the bottom piece for the planter box using a circular saw, using the measurements of the ledge's width and depth. Drill a few 1/4 inch holes in the bottom of the board to assist in drainage.
Cut two side pieces that are the same height as the front and back piece, and the 2 inches less than the depth as the bottom piece.
Drill pilot holes in the front and back pieces where they will meet the side pieces. Drill two holes on the bottom piece at each end as well.
Glue the front and back pieces to the two side pieces and clamp them together. Allow the glue time to dry before moving the pieces.
Add a bead of glue on top of the bottom piece around the edgesand set down the front, back and side pieces on the bead. Push down and wipe away any glue with a rag. Allow the planter box to dry.
Screw the pieces together with 1 1/2-inch galvanized screws where the pilot holes were drilled.