First of all, you need to have money to buy them; you can get it by selling wood, for example.
Once you have it, go to the clothing store at night and buy it. Then go home, and in the closet in the bedroom, you can interact with it to dress Rosette in whatever you want. There are certain outfits that are automatically triggered in specific scenes or actions—for example, you’ll find the black lingerie if you go to Stela’s house at a certain time.