Determine the width of the headboard at the widest point using a measuring tape. Use a pencil and straightedge to mark the exact center. Use the measuring tape to measure out from the centerline and mark the cut lines. Use a straightedge to keep the lines straight. For example, if you want to convert the king headboard to a full, there is a 22 inch difference in width. Measure 11 inches on either side of the centerline and then mark the headboard from top to bottom.
Cut the headboard along the two cut lines you drew using a circular saw. Keep the cuts as straight as possible. Remove the center cut out section leaving two identical pieces from the cut headboard. Sand the headboard using medium-grit sandpaper to remove any splinters or rough edges.
Turn the headboard face down on your work surface. Since you cut the center out of the headboard, put the two cut edges of the headboard flush against one another. Attach three metal brackets to the back of the headboard using screws. Place the first bracket 3 inches from the top edge of the headboard and the second 3 inches from the bottom edge of the headboard. Place the third in between the first two. Position the brackets so they stretch across the center seam where the two pieces of the headboard meet.
Measure the height of the headboard.
Mark the cut lines on the headboard to indicate how much you want to cut off. You can cut from the legs to make the bed closer to the ground or from the top to make it easier to fit under a window. The reason you need the bed cut down will determine where to cut from.
Cut across the cut lines with a jigsaw. Sand the exposed edges using medium-grit sandpaper.