To doll up is to dress up or make someone look fancy, beautiful, or elegant. If you doll up your friend for the prom, you might curl her hair and do her makeup.
You can doll up yourself by primping before a special event — dressing carefully, applying lipstick, styling your hair, or wearing jewelry. Children sometimes doll up their stuffed animals by putting them in doll clothes, and you can also doll up a room by decorating it with wallpaper, pillows, slipcovers, and accessories. When you doll someone up, you dress them almost as if they were an actual doll.