To replace the upholstery fabric on car seats, start by unbolting and removing the seats from the car, and then remove the old upholstery from the seats. Be sure not to damage, them because you're going to need them intact later.
Once the old upholstery is fully removed from the seats, use it as a guide to draw and cut out upholstery pieces from the new material you selected. Sew the separate upholstery pieces together, making sure to closely mimic how the old upholstery was done. After you finish sewing, install the new upholstery onto the seats. Return the reupholstered seats into the car, and retighten the bolts that secure them in place.