You’ll need 1/2 yd of oil cloth, matching thread, 1 yd shock cord, a couple of grommets and a toggle.
Grab a piece of paper, flip your bike upside-down and trace your seat. Add 1/4 to 1/2 in for seam allowance.
Cut out your seat shape and a strip that is 4 in wide and as long as the perimeter of your seat. This one came out to be 4in x 3oin. Bust out the string trick to make your perimeter measurement.
Install grommets in the ends of your strip so that your shock cord can come out the back of your seat cover and connect through the toggle. Insert your shock cord, fold over the oil cloth and pin in place. Sew up this casing along the entire strip.
With right-sides together pin the unmodified edge of the strip to the bike seat shape. Make sure your grommets line up with the back of your seat. Sew to attach, add a toggle to your shock cord and you’re done!