On a flat surface, position fabric on chair seat, right side up; pin lightly to chair cushion. Carefully flip over the cushion and fabric to keep the fabric in place.
Trace outline of seat onto the back side of fabric, adding 4" all around; cut out.
Start from the middle of one edge: Pull fabric tightly up to the bottom of the seat; staple in place. Repeat on opposite side, then on remaining sides, then staple from the middle outward. At corners, fold fabric neatly; staple in place.