Start out by thinking about the type of person Atticus was. He was a lawyer, kind, and usually calm. His letter would likely be well written with a careful eye toward word choice. He certainly wouldn't use any abrasive language or hateful rhetoric to prove his point. Atticus usually makes a point by remaining calm and rational. He would probably give examples of specific points of racism in Maycomb. He might also talk about his own experiences with racism, like the neighbors' reactions to Calpurnia's influence on his home and his children or his involvement in the trial of Tom Robinson. Atticus would probably offer evidence of how the color people of Maycomb were no different. Just remember that Atticus is a lawyer and likely to stick with a calm dissemination of the facts and your letter should turn out fine.