Slide the old cover off a vintage butterfly chair. Trace around it with a pencil on paper to make a pattern for the new chair. Add 1 inch around the perimeter for the seam allowance. Be sure to include the pockets when creating your pattern.
Pin the seat cover pattern to the heavy weight fabric and cut it out. Use the pattern to cut pockets from the heavy fabric.
Pin the seat-cover pattern to the backing fabric. Cut it out.
Hem the bottom edges of the corner pieces using a needle and thread.
Baste the piping onto the heavy fabric. Baste the corners pieces to the bottom fabric
Pin the heavy fabric top piece to the cotton fabric bottom piece with the right sides together. Sew around the perimeter of the seat cover, leaving a 3-inch opening to turn the fabric.
Stitch around the corners a second time to reinforce the seams.
Turn the cover right sides out. Hand stitch the opening closed.
Place the seat cover onto the frame. Sit in it for a few minutes to stretch the fabric and test it for safety.