Measure the top of the pergola and add 3 inches to the measurement on each side for hems. Cut the fabric to fit.
Hem all four sides of the fabric. Fold the fabric under 1/2 inch and iron the fold. Working in the same direction, fold the fabric again to make a 2 1/2-inch hem. Pin the hem in place and then sew it closed.
Use a grommet tool to add grommets to all four corners. A grommet is a circular piece of metal that reinforces a hole in fabric. The grommet tool presses the metal into place. If your grommet tool doesn't automatically remove the fabric in the center, make an "X" in the fabric with a sharp knife and then cut out the fabric.
Install eye hooks on the corners of the pergola, about 2 inches below the top. Drill pilot holes and then screw the hooks in place with the open part of the hook facing the ground.
Lay the fabric over the top of the pergola and slide the hook on one end of a bungee cord through the hole in the corner of the fabric. Hook the other end of the bungee cord through the matching hook eye.