Identity and family relationship records, a clear statement of the exact proposal under consideration, and specific questions about eligibility, funding, and the currently posted official instructions. Any subsequent USCIS forms and supporting evidence should be prepared according to the official directions received.
Rely on a specific, dated version of the official instructions
The official site describes further USCIS instructions being issued after an initial payment step. Review those requirements directly before assuming that documentation prepared for a different program can simply be reused unchanged. Keep a record of exactly which version of the instructions informed the plan, since the framework has been described as subject to change.
Expect identity and civil documents for every person included, a full immigration and travel history, background and security information, and evidence going to the classification being used. Keep passports, birth and marriage certificates, prior U.S. immigration records, and name-change documents orderly, but do not assume that every standard immigration document is automatically requested.