Drape the fabric, with the wrong side out, over the back of the folding chair. Center the fabric on the chair, then use the sewing pins to outline the chair back. Pin from the top down following the line of the chair to the end of the fabric on one side and then the other.This will give you an outline, or pattern, to sew around. The fabric should be snugly, but not tightly, pinned. The fabric should lift easily off the chair after pinning.
Remove the pinned fabric from the chair back. Using a sewing machine set on a straight stitch, sew in a continuous smooth line along the pattern created by the pins. Remove the pins as you sew.
Place the cover on the chair, wrong side out, to check the form. The cover should fit snugly so it does not slip around on the chair, but not so tight that it is difficult to put on or take off.
Use the scissors to cut away any excess fabric from the top or sides of the cover, leaving a 1/2-inch seam allowance.
Use sewing pins to mark the bottom hem of the chair cover at the length in both front and back that you desire. Remove the cover from the chair.
Measure and mark with pins the hem plus an additional 1 inch on the front and back of the chair covers. Fold over the raw edge 1/2 inch. Fold the edge over another 1/2 inch and pin in place. Sew the hem down with a straight or decorative stitch on the top edge of the fold. This will give the cover a nice, finished look.
Turn the cover right side out and place it on the chair.