In the Middle Ages, serfs were owned by landowners, and did menial work, often on the land. You could obtain a hessian sack, and cut holes for the head, and two armholes, then put it over the head and tie it around the waist with thin rope or thick string. To complete the look, you would need to have bare feet and sandals. Hope this helps. Whatever you decide to go dress as, I hope you have a good time.