In To Kill a Mockingbird, how does Aunt Alexandra help contribute to Scout's development from a tom boy into a lady?
Scout clashes with Aunt Alexandra for most of her early childhood. Scout sees her as stuck-up and thinks she does not understand children. After Aunt Alexandra comes to live with them, things actually get worse for a while because Alexandra tries to turn Scout into a lady by making sure she wears dresses, attends social gatherings of the ladies. She tries to encourage Scout to appreciate her heritage, but focuses at first on how the Finch family is better than everyone else. This is exactly the opposite of the teachings Atticus has given her. Atticus has taught her to respect everyone, no matter how poor or what race.
Aunt Alexandra does play a pivotal role in Scout’s coming of age though. As Scout matures, she comes to understand Alexandra. She realizes that Alexandra actually does not approve of some of the bigoted remarks that the ladies make, and that she does worry about her brother. Ultimately, it is their love of Atticus that unites them and allows Scout to see that Alexandra does come from a place of having Scout’s best interests at heart. She is trying to teach Scout how to survive in the world that exists, and Atticus is trying to teach her to change it, or at least not accept it. Ultimately, both are lessons she needs to learn.
I linked to a similar question in the TKM group, but it's not exactly the same question as yours, so I'll supplement it with some additional information.
Scout grew up without a really feminine role model, aside from Miss Maudie, who did not live in her house. Calpurnia is certainly a woman, but not the type of woman accepted as feminine in the South at the time in which the novel is set. Before Alexandra came into the home, Scout was allowed to be like Jem. She didn't have to wear dresses all the time, learn proper posture and manners, or act particularly girly because Atticus just wanted her to be smart and well-behaved. He was liberal enough that it either didn't bother him that Scout didn't fit into the social standards of femininity, or he didn't know exactly how to handle her boyish behaviors. When Alexandra comes into the picture, however, she takes control of the situation and makes an effort to make Scout act socially like a girl is supposed to according to the social standards in Maycomb at the time.