REPLACE THE PLYWOOD IF NEEDEDCheck out the wood and foam that’s underneath to see if it’s in good enough condition to reuse or not. If it need's to be replaced, use the old plywood as a pattern to cut new 1/2″ plywood using a jigsaw.
Cut the excess fabric about 1/2" down from the fold. Do not cut too close to the seat. Pull the flap of fabric toward the bottom chair and tuck the excess into the fold as you pull it. Pull tight and staple. Repeat for the other 3 corners.