Measure the footboard to find the center, and mark it. Use a saw to cut the footboard in half.
Line up the cut ends of the footboard with the front corners of the headboard so that the bottom-most wood pieces are even. Mark where you’ll attach them. Drill holes through the headboard and into the cut sides of the footboard.
Place a small dab of glue where the pieces will be attached. Using wood screws that are long enough to go through the thickness of the headboard, attach both halves of the footboard to the headboard. Countersink the screws and cover the screw heads with wood putty.
Measure the distance between the two halves of the footboard. Cut a piece of lumber, either new or scrap, to form the front of the seat’s base. Using wood glue and wood screws, attach it to both halves of the footboard so that it’s even with the bottom-most cross-pieces, using a level to make sure it’s even. If your headboard doesn't have these pieces, or braces, measure and cut pieces of lumber and attach them to the inside to hold the seat; you should end up with a rectangle of wood to which the seat boards can be attached. Cover screw heads with wood putty and use L-brackets to reinforce the corners.
There are a few ways to make the seat: by using a solid piece of plywood, by running lumber lengthwise or by running lumber front-to-back. Make your decision based on what you have lying around or on how deep your bench-seat area is. For instance, if the depth is an even 36 inches, six 1-by-6s running lengthwise will fit nicely; if it’s 30 inches, use three 1-by-10s.
Another option is to use a solid piece of plywood. Measure the depth and width of the bench between the bottom-most braces, and cut a piece of plywood to shape, measuring and cutting out any notches to accommodate the four corners.
Yet another option is to run pieces from front to back to form the seat. Use several 1-by-3s, 1-by-4s, or whatever you have lying around cut to the correct depth of your seat. Use as many as is necessary to fill the seat area without significant gaps between them.
Apply a thin line of wood glue along the bottom-most side braces for a seat running lengthwise, the front and back braces for seats running front to back, or all four for a plywood seat. Use wood screws to screw the lumber for the seat into the bottom-most braces. Cover screw heads with wood putty.
Using sandpaper, smooth the wood putty and rough up the finish on the rest of the bench. Wipe it with a damp towel to remove sanding dust.
Apply one coat of primer and allow it to completely dry. If the bench is to be used outdoors, be sure to paint the underside as well.
Apply at least two coats of paint, sanding lightly between coats for better paint adhesion. Wipe off after sanding.
If you choose, you can apply a sealer to help preserve your paint. This is an especially good idea for headboard benches that will be placed on a porch.