Trim the edges of a 28-by-54-inch piece of 2-inch-thick foam to a 45-degree angle using an electric knife to start your queen-sized headboard. Iron flat a piece of fabric around 6 to 8 inches larger all around than the foam. Lay the fabric wrong-side up.
Spray the front of the foam (the side with the angled edges) with spray adhesive, following the adhesive manufacturer's instructions. Lay the foam down carefully over the back of the fabric, then flip the foam over and smooth out the fabric so that it is seamlessly stuck to the foam.
Fold the edges of the fabric over the back of the foam, making neat "hospital corners" at the corners, then hand-baste the fabric to the back of the foam with needle and thread.
Pin the flange edge of cord piping to the back of the foam, so that the cord piping sits around the front edge of the foam. Stitch the piping flange to the back of the foam and remove the pins.
Trim the ends of two planks of 60-inch panel molding and two planks of 34 inch panel molding to 45 degrees so that you can form a frame from the molding. Paint the molding in the desired color. For a real boudoir feel, apply a gold glaze over the molding, or distress it for a shabby-chic boudoir. Glue the panel molding frame together at the corner joints and let dry.
Lay a piece of 31-by-54 inch 1/2-inch-thick plywood flat on the work surface. Spray adhesive on the back of the foam, then set the foam on top of the plywood exactly centered. There should be an extra 1 1/2 inches of plywood overall around the foam.
Fit the molding frame around the foam, ensuring the cord piping sits up between the edge of the foam and the molding. Secure the frame to the edge of the plywood with finishing nails hammered every 4 to 5 inches around the perimeter. Further secure the foam to the plywood with finishing nails hammered right at the edge between the cord and the foam, so that they remain invisible.
Touch up any nail holes with paint to finish your French boudoir headboard.