Use needle nose pliers if repairing an off-track zipper slider on a zipper with a bottom stop. Remove the metal stop with the pliers. Discard the stop. Slide the zipper slider off the zipper teeth until it is free.
If the zipper has no bottom stop, rip any stitches that bind the end closed with a seam ripper. Slide the zipper slider off the zipper teeth until it is free.
Align the zipper teeth along the length of the cushion opening and pin the opening together.
Slide the slider onto the teeth until the teeth interlock. Ease the slider along the track until about ½ inch of closed zipper extends. Grasp the closed zipper end with the needle nose pliers in one hand and pull the zipper slider with the other. Continue closing the zipper until the cushion cover remains about two-thirds open, removing pins as the zipper slider slides along the track.
Hand stitch a bar tack over the closed zipper end to temporarily hold it in place.
Cut a 1 ½-inch length of twill tape. Lay the twill tape across the bar tack and pin it in place. Machine-stitch the twill tape to the cloth zipper tape as close to the zipper teeth as possible. Remove threads and turn the cushion right side out.