Identity and civil documents for every person to be included, a written record of the source of the gift and fee funds, the household's complete immigration and criminal history, and a dated copy of the official instructions. USCIS then specifies further forms and evidence after the initial step.
Prepare the file the authorities will ask for after the fee
The official process describes further USCIS instructions following the initial application, so the applicant should expect to produce the ordinary immigrant-petition evidence for whichever classification is assigned, plus whatever the program itself requests. Assemble in advance: passports and birth and marriage certificates for each included person; police certificates from every country of residence as required; a record of every prior U.S. entry, visa, refusal or overstay; and the financial trail behind the gift and fee, since vetting can reasonably ask where the money came from.
Keep a dated printout of the page and FAQ that informed the decision, because a later change in terms is easier to discuss when the version you relied on is on file. Hypothetical example: an East Hants farm owner intends to pay the gift from the corporation; whether that is permitted under the individual route, and how it is taxed, are questions for the official instructions and a tax adviser before the transfer.