1. To determine the dimensions of the shelves, measure the depth and width of the window frame, and subtract 1/2 inch from the width.
2. Have a glazier cut a 1/2-inch-thick piece of glass to size for each shelf; for a more finished look, have the edges sanded. Using a level and a ruler for precision, make pencil marks where each shelf support should go, starting from the top of the window frame.
3. Make supports out of molding, available at hardware stores: Using a hand saw, cut two lengths of molding for each shelf (the molding length should equal the depth of the frame). Sand the ends smooth.
4. Drill three evenly spaced holes (just bigger than the head of a wood screw) in each support. Hold a support against the appropriate mark on the window frame, insert the bit of an electric drill through one of the holes, and drill a starter spot into the frame. Repeat for the other holes, then countersink screws so the heads don't show. Repeat for remaining supports. Fill holes with wood putty, sand smooth, and paint supports. Once paint dries, attach a felt dot or plastic glide to each support end, and set glass shelves in place.