Measure each side of the window. Transfer the measurements to 1-foot-by-3-foot slats of wood. Cut the wood to size with a circular saw.
Apply wood glue to each of the outer edges of the short pieces of wood. Put the pieces together to form the shape of a rectangle; the edges of the shorter pieces of wood should butt up against the top sides of the longer pieces of wood.
Secure the frame by hammering finishing nails through the top and bottom of each of the outer edges of the longer pieces of wood, driving the nails into the edges of the shorter pieces of wood.
Place prefabricated coffee table legs into each of the corners of the frame; the tops of the legs should be flush with the tops of the frame. Drill 2-inch wood screws through each of the outer edges of the frame and into the coffee table legs. Set two screws through each edge; one near the top of the frame and one near the bottom.
Apply wood glue around the perimeter of the window frame. Set the window on top of the table frame, glue-side down.
Hammer finishing nails every 6 to 8 inches along the perimeter of the window frame, driving them into the table frame.
Place the window on the wall behind the bed. Position the window until you are satisfied with its placement.
Drill 3-inch wood screws through the frame of the window and into the wall. Place the screws every 8 inches along the entire perimeter of the window frame.
Conceal the heads of the screws with wood putty that matches the color of the window frame.