Remove the cushions from the lawn chairs. Cut off and remove the old fabric exposing the foam cushion inserts, if desired. You can also cover the old fabric with new.
Measure the lawn chair cushions, noting the length, width and depth. Double the depth measurement and incorporate it into the width and depth measurements. For example, if the cushion measures 16 inches long and wide and the depth is 6 inches, you will end up with a measurement of 22 inches long by 22 inches wide, when you double the depth and add it to the other measurements.
Add an additional 2 inches to the length and width measurements to allow room for the seams. For example, add 2 inches to your length measurement of 22 inches and the width measurement of 22 inches, ending with your final measurement of 24-by-24-inches.
Double your outdoor upholstery fabric and spread it out on a flat surface. Mark your final measurements on the fabric using a piece of chalk.
Pin the fabric together to keep it from slipping. Cut out your panels for the outdoor lawn cushions following your chalk marks.
Remove the pins and turn the fabric so the right sides are facing each other. Pin together again.
Sew three sides closed with a 1-inch seam and allow the forth side to remain open. Double stitch the beginning and ending stitches to keep the seams from pulling apart.
Remove the pins and turn the upholstery covering so the right sides are facing outward. Slip the lawn chair's cushion inside the fabric covering.
Turn the remaining seam inward and pin it together, slightly overlapping one seam edge over the other. Thread a heavy-duty needle, doubling the thread and sew the seam closed. Snip off any loose threads and place the cushion back in place on the lawn chair.