Staple or tack the vinyl to the chair seat, leaving the vinyl loose 1 inch on each side of the corner.
Pull one side of the corner taut with a crisp edge from the end of the staples about 1 inch past the corner point. Staple this edge to the chair seat.
Pull the other side of the corner taut to create a crisp edge. Fold the vinyl to the backside so the corner is smooth and the excess vinyl does not show when you flip of the seat. Make a pleat fold at the corner if the vinyl shows on the right side. Staple the or tack the vinyl to the backside of the seat.
Repeat these steps on each corner to secure the vinyl to the upholstered cushion.
Cut off excess vinyl on the backside of the seat cushion to a distance of 3/4- to 1-inch from the staples.
Attach the seat cushion on the chair after securing the corners. Trim additional vinyl from underneath the seat if it does not lay flat on the chair frame.