Sharpen your pencil. Turn off any distractions. Get ready to write lots of answers to lots of essay questions. Search your memory, consult old copies of your resume--be prepared to respond with verifiable details about your 20+ years in the paid workforce.
Keep in mind that your most expertly crafted answers will never be enough: there will be yet another hoop to jump through. The director at my local Encore office seemed determined to map my every moment since 1999, as if to establish the provenance of weapons-grade plutonium or an antique Grecian vase. There were very few openings for Encore Fellows in my region (some applicants weren't placed for at least a year), so perhaps the multiple rounds of required essay questions were intended to weed out the less committed in the applicant pool.