What is Tally's plan at the end of the novel to save Shay and to help save people from the brain lesions and the control of the specials?

Tally's plan is to return to the city, where she can be turned into a pretty. Afterward, she plans to return and become the guinea pig for testing the cure for the brain lesions. Just to make sure all goes as planned, Tally writes herself a letter reminding herself of her intent.