Measure the length and width of the folding chair's seat.
Cut out a piece of decorative fabric that matches the width of the seat plus 2 inches and is twice the length plus 3 inches. For example, if the seat measures 12-by-12 inches, the fabric should measure 27-by-14 inches. The extra fabric will allow for seams and the thickness of the pad.
Fold the fabric in half with the face inward, making it slightly larger than the seat size.
Sew seams along the right and left edges to join the fabric together. This closes three sides of the cushion when the folded edge is included.
Cut four 12-inch pieces of decorative cord that complements the cushion fabric.
Sew the ends of two cords to each corner next to the open side, extending away from the cushion.
Turn the cushion right side out so it resembles a bag with four cords extending from the open end.
Cut a piece of 1-inch upholstery foam to match the chair seat size. Slide this foam into the cushion fabric.
Close the open edge of fabric over the foam, pinning the seam with straight pins. The edges of the fabric should be tucked inside of the cushion.
Sew the final seam closed with a slip-stitch.
Place the cushion on the chair seat with the folded edge facing the front. Tie the cords to the sides of the chair to secure the cushion in place.