Remove the old seat cushion from the chair. Turn the chair over. Loosen the screws attaching the chair frame to the hard cushion panel. Remove the screws and lift the cushion from the chair.
Turn the cushion upside down. Use a staple remover, screwdriver or chisel to remove the staples from the paper or cardboard facing and the cushion panel, if the paper is there. Put the paper aside.
Remove the cushion from the panel. Cushion and chair styles vary, but the upholstery fabric typically fastens to the panel with upholstery tacks or staples. Remove them with a screwdriver, chisel or hammer.
Remove all old cushion and upholstery fabric, filling and batting. If the upholstery is glued, cut off as much as possible, or use a glue remover to loosen it.
Set the new upholstered cushion upside down on a clean, flat surface. Place the chair panel, bottom facing up, on top of the cushion. Line them up so all edges are flush.
Press down on the panel, and staple the bottom of the perimeter to the cushion using a staple gun with upholstery or heavy-duty staples. Staple about 1 inch from the edge.
Turn the cushion over to check that it's secured properly. Look for pockets where you can add a staple on the underside.
Glue the cushion to the panel for a staple-free, but more permanent, option.
Place the cushion and panel atop the chair frame, and install the original screws tightly. Turn the chair right-side up.