Measure the width of your mattress and the slab door. The door should measure a few inches taller than your bed is wide. If it is too long, mark a trimming line with the pencil.
Lay the door on a flat working surface and mark a straight trimming line across the door using the yardstick and T-square, as necessary.
Trim the door, as needed, across the trimming line using the power saw. Sand down the rough, trimmed edge as desired using sandpaper.
Decide which long side of the door will be the top of the headboard, the trimmed or non-trimmed side, and saw a length of the chair rail molding so it is the same length across.
Lay the chair molding across the top edge of the top of the headboard, and then glue it into place using the wood glue. Allow the glue to dry, following the manufacturer’s instructions, and then secure the molding further with a few evenly-spaced wood nails hammered flush against the surface.
Determine the placement of your headboard. You will hang it, like a heavy picture, so it is half visible above your pillows. Mark your wall lightly with pencil for the picture hooks.
Flip the headboard to its backside and drill two holes, one on each side of the headboard, directly below the molding. Screw a D-ring into each hole.
Thread some picture wire between the D-rings and secure by twisting the ends with pliers. Make sure to leave a small amount of slack in the wire.
Hammer picture hooks into your wall, along the marked line, and then hang your headboard onto the hooks by the wire.