Draw the shape of the front piece of the crown on half-inch plywood. Choose a shape that appeals to you, such as a scroll pattern, an actual crown or any other random design. Keep the shape symmetrical. Make the height 6 to 10 inches, depending on the shape you choose and how tall you want the crown to be. Make the width 6 inches smaller than your mattress width. For example, if the mattress is a twin, make the shape 33 inches wide.
Cut the shape out, using a jigsaw. For tighter turns, use a rotary tool. Sand the edges to smooth them, using medium-grit sandpaper.
Cut two sidepieces for the crown from half-inch-thick plywood, using a circular saw. Make the height equal to the height on the sides of the front piece. Sand the edges of the plywood.
Apply wood glue to the edges of the sidepieces that will attach to the front piece. Place the sidepieces against the back of the front piece, along the outside edges. Check the alignment to make sure the edges are squared. Hammer nails through the front piece into the 1/2-inch sides of the sidepieces. Place two nails per sidepiece, spacing the nails 1 inch above and 1 inch below the top and bottom edge.
Secure the sidepieces further by attaching L-brackets in the corners where the sidepieces meet the front piece. Use half-inch wood screws, placing one screw through each hole in the brackets. Because the brackets are about a fourth-inch thick, they won't go through the plywood.
Paint all sides of the plywood pieces, using spray paint or flat wall paint. Ensure that you have full coverage with the paint, applying additional coats as needed. Let the paint dry overnight.
Attach two pieces of fabric to the backside of the front and sidepieces, using a staple gun. Extend the first piece of fabric from the edge of the sidepiece to the center of the front piece. Extend the second piece from the other sidepiece to the center of the front piece. Gather the fabric in pleats, and staple as you go. Choose any fabric that complements your decor. The length depends how high you want to hang the crown.
Cover the staples with ribbon. Cut a piece of 1-inch ribbon long enough to stretch from the back of one sidepiece, across the front and around to the back of the other sidepiece. Apply hot glue to the back of the trim, and press it over the staples.
Place a picture-hanging hook an inch down from the top edge of one sidepiece. Secure the hook, using a 1-inch wood screw. Place a second hook on the other sidepiece in the same position.
Drill two one-third-inch holes in the wall where they will meet the hooks you added to the sidepieces. The height on the wall is up to you. Squeeze the spring-loaded toggle on a spring toggle, and push it through the first hole. Once it's through, pull the screw toward you until the toggle is resting on the other side of the wall. Twist the screw to tighten it until the screw head sticks out a half inch. Repeat the process with the second hole and another spring toggle.
Hang the crown on the screws, using the hooks. Set the bed up below, and pull the fabric on either side of it.