How does Atticus prepare Scout for the trial?

Atticus prepares Scout for the trial in a number of ways. For one thing, he tells her the reasons behind his taking on such an unpopular case. Everyone is equal in the eyes of the law, and Tom Robinson is no different despite the prevalence of racial prejudice in Maycomb. He could never be proud of himself again if he didn't take on the case, nor would he be able to expect Scout or Jem to trust him anymore. Atticus spends so much time telling his children how important it is to do the right thing, and he must lead by example.
Atticus, as always, is incredibly open and honest with Scout. He tells her in no uncertain terms that the Finch family might well face a lot of trouble in town over his decision to represent Tom. Scout is going to hear all kind of horrible words spoken about her father. But she needs to stay strong and show courage; she needs to rise above the insults and not let other people get to her. In a response to a question from Scout, Atticus also explains to her what rape is. This shows once more how Atticus treats his children like adults, trusting them to show maturity in understanding some of the more unpleasant aspects of life.