King-Size Framed Upholstered Headboard

Cut (or have cut) the 3/4" back to 76-1/2" x 40". Builder's-grade plywood is fine here since it's fully covered, but pick a flat, undamaged sheet — a bowed back telegraphs through everything.

Fasten the top and bottom trim (1x3 at 76-1/2"), then the side trim, to the back with 1-1/4" nails and glue, keeping outside edges flush. This layer builds the frame's depth.

Fasten the bottom rail (76-1/2", not the diagram's 82-1/2") with 2" nails and glue into both the back and the trim layer; the front overhangs 1". Add the legs, then the top rail (78", not 84"), nailing into the legs, top trim, and back. Fasten the leg fronts through the sides and to the headboard top.

Measure and cut the store-bought molding to frame the opening, mitering the corners at 45°. Glue and nail it with 1-1/4" nails. Fill nail holes and any miter gaps, sand, and finish the wood now — before the fabric goes in, so you don't get finish on the upholstery.

Cut the 1/2" panel to about 35" x 71-1/2" and dry-fit it in the opening, leaving clearance for foam and fabric. Lay foam on the panel, wrap the fabric around, and staple it taut on the back — pull corners neatly so they don't bunch. Set the wrapped panel into the opening and screw it to the back from behind with 1" screws.