Turn the chair over so you can reach the underside where the seat is held on with screws. Use a screwdriver to remove the screws. Set the screws aside and pull off the loose seat.
Use a flathead screwdriver to pry up the tacks or staples holding the fabric in place. Once the fabric is loose, set it to the side. Remove the old cushion. If it was glued in place, pull off as much as you can and scrape the rest off with a putty knife.
Place the bare seat board on top of a 1- to 2-inch foam sheet. Trace the shape of the board on the foam and cut it out with a utility knife.
Cover the top of the board with spray adhesive. Lower the foam padding onto the board and let it dry.
Replace the fabric over the top of the padding, and pull the edges back underneath the board. Staple the fabric back into place with a staple gun. Replace the seat on the chair and replace the screws holding it in place.