Measure the width of the full-sized headboard. Full-sized mattresses measure 54 inches wide, so the headboard should be at least 54-inches wide. Cut three 8-inch-wide by 2-inch-thick wood planks equal to the headboard width. Three 8-inch-wide wood boards will make a 25-inch-deep bench seat if you separate the boards by half an inch. You can adjust the depth of the seat by using more or fewer 8-inch-wide planks or by using planks with different widths.
Position the three planks in a vertical row. Make sure that the ends of the planks are flush. Allow up to half an inch of space between the center plank and each end plank.
Attach a 24-inch-long metal bracket across the width in the center of the three planks with wood screws. Attach a metal bracket across the width at each end of the three planks. Place the end brackets 4 inches from the outside edge of the planks. Center and attach another metal bracket between the center bracket and each end bracket. These three joined planks make the bench seat.
Attach two decorative chair legs to the front left and the front right corner of the bench seat with wood glue and wood screws. On the same front edge of the bench seat, attach a decorative chair leg to the right of the metal bracket that is to the left of the center bracket. On the same front edge of the bench seat, attach a decorative chair leg to the left of the metal bracket to the right of the center bracket.
Attach two decorative chair legs to the back edge of the bench seat in line with the legs to the left and right of the center bracket. The attachment posts of the full-sized headboard work as the back corner supports of the bench seat.
Attach the back edge of the bench seat to the front of the full-sized headboard with L-brackets. The decorative chair legs and the headboard posts should be flush so that the bench sits level. Place an L-bracket at each end of the bench seat, in the center of the seat and equally spaced between the center and the end. Line up the L-brackets so that they do not interfere with the metal brackets on the underside of the bench seat or with the chair legs.