Take a piece of the backing fabric (scrap) and mark some pencil marks on it. Try everything you can think of to erase them and see what works best. Some people use a Mr Clean magic eraser but I'd only use that on white in case it bleaches it. Some swear by a mild mixture of water and dawn clear dishwashing soap. If you type in removing pencil marks from a quilt in Google you might get some other suggestions. Always try suggestions on a scrap first and not on your quilt!