Start from the official program page and FAQ, which set out the current requirements, and expect identity, background and financial documentation. Assemble proof of the lawful origin of the funds regardless, because that question is asked in every serious process.
Prepare the records, verify the list
Hypothetical example: a Grand Bay-Westfield applicant asks what forms to fill in and is given a list by an intermediary. Any list of that kind should be checked against the official program material before a fee is paid to anyone, because the framework is new and published requirements have changed. What can sensibly be prepared in advance is the documentation nobody escapes: passports and identity records, immigration and travel history, police and background records where they exist, tax filings, and a documented trail showing where the money came from.
For a retired professional funding the payment from a pension commutation, a house sale or long-held savings, that trail is straightforward but takes time to gather. Prepare it, and verify the actual filing requirements from the official source rather than from an adviser's summary.