Cut two pieces of 3/4-by-33-by-40-inch plywood to create the sides of the bed. Sand the cut edges using medium-grit sandpaper.
Paint or stain the plywood pieces. Let the paint or stain dry overnight. Apply a second coat, if necessary, for full coverage.
Apply paint stripper to the surface of the king headboard using a paintbrush. Follow the manufacturer’s instructions and let the stripper sit for 15 minutes or as indicated. The stripper will cause the paint or sealant on the headboard to crack and wrinkle. Once this happens, rub the headboard with steel wool to remove the paint or sealant. Repeat the process as many times as necessary to remove all the paint or sealant from the surface of the headboard.
Clean any remaining paint, sealant and stripper from the headboard by applying a lacquer thinner. Because lacquer thinners vary by manufacturer, follow the directions on the product for proper application. Let the headboard dry for 48 hours.
Paint or stain the stripped headboard using the same color you used for the plywood pieces. Use a second coat if necessary. Let the headboard dry overnight between coats.
Place the headboard flat on your work surface with the side you want against the mattress facing up. Apply wood glue on the 3/4-inch edge running along the 33-inch length of one of the plywood pieces. Align the glued edge on the outside of edge of the side of the headboard. Press the glued side against the upward facing side of the headboard, making sure the bottom and side edges are flush.
Place a metal L-bracket against the plywood sheet and headboard where they meet. Position the L-bracket 3 inches from the bottom of the plywood and headboard. Secure it in place using wood screws. Place one screw through each of the holes on the bracket. Repeat the process with a second L bracket 3 inches below the top edge of the plywood.
Repeat the process of gluing and screwing on L-brackets with the other piece of plywood on the opposite side of the headboard. Use two more L-brackets and more wood glue to secure it.
Set the complete daybed frame upright and position it where you want it in the room. Slide a twin metal bed frame in between the plywood pieces, resting the length of the bed frame against the the king headboard of the daybed. Set up your box spring and mattress as normal.