Place a large piece of paper on the floor. Flip the bar stool over and set the seat on the paper. Trace the bar seat with a pencil to make a template.
Measure the height of the seat, from the cushion top edge to the bottom. Add 2 inches to this measurement and increase the diameter of the traced circle by this amount.
Cut out the adjusted paper template, align it on the decorator fabric and pin to hold it in place. Cut out the fabric circle.
Measure the circumference of the fabric circle and multiply the value by 0.75. Cut a piece of 1/4-inch elastic to the length of the adjusted measurement.
Turn the edge 1/4 inch to the wrong side and iron or pin to hold in place. Turn the edge an additional 1/2 inch to the wrong side and iron or pin to hold it in place.
Sew close to the inside edge to hold the fold in place and create a casing for the elastic. Leave a 1-inch hole to feed through the elastic.
Attach a safety pin to one end of the elastic and push it through the casing opening. Push the safety pin through the casing, pinning the opposite end to the edge of the casing to prevent it from pulling through.
Remove the safety pin after feeding the elastic through the casing. Match the two ends of elastic and sew them together to secure. Sew the 1-inch opening closed to finish the cover.