Measure the width of your window. This is the length of your flower box. If you have a particularly wide window, build two window boxes and place them side by side.
Mark the window width measurement on two boards with a pencil.
Cut the two boards at this measurement, using a circular saw.
Mark and saw two 10-inch-long boards.
Create a frame-shape with the four cut boards. The two longer boards are up the front and back, while the 10-inch boards fit inside the two long boards at each end.
Drill four to five screws through each of the four ends of the longer side boards and into the sides of the shorter end boards.
Place the wood frame onto a piece of plywood (½ or ¾ inch thick). Mark all the way around edge of the wood frame onto the plywood with pencil.
Cut the marked plywood with a circular saw.
Put the cut plywood on top of the wood frame (top and bottom are the same, and drill a screw about every 4 or 5 inches through the plywood into the frame.
Drill holes across the plywood at random distances. This will allow the water from the soil in your window box to drain out easily.
Turn your window box right-side up, and paint, stain or use a clear polyurethane to seal it. Let dry thoroughly.
Screw eye hooks along the top of the back board of your window box, about every 6 inches.
Screw open hooks onto your house, under the window, at the same intervals as the eye hooks on your window box.
Hook your window box eye hooks onto the open hooks of your house.