After you find Rosabeth between Majula and the Shaded Woods, you will need to unpetrify her with a fragrant branch of yore. Following this she will confess her embarrassment at having nothing to wear and you will be given the option to give her some clothing. Give her any piece of armour you choose and the achievement will unlock. Next time you are in Majula Rosabeth will be wearing the items that you give to her.