Identity and background documentation of the kind any United States immigration process requires, plus clear records of the source of the funds to be given. Beyond that, follow the official programme instructions rather than a checklist prepared for a different category, and use official payment channels only.
Document the money and use official channels
Two categories of document matter. The first is personal: passports, civil documents, and the background material any immigration process asks for. The second is financial, and it deserves particular care.
A large payment to a government has to come from somewhere, and being able to show clearly where it came from protects the applicant in more than one forum, including on the Canadian tax side. Keep the records of the sale, distribution or account withdrawal that funds it. On mechanics, use only the payment channels the official programme identifies.
New programmes attract intermediaries offering to handle payment or to guarantee an outcome, and a payment routed through a third party is a payment that can disappear. Nobody outside the government can guarantee an immigration result. If an adviser's confidence exceeds what the official page says, that gap is the warning.
Finally, expect the required documentation to be adjusted as the programme is administered, and check before assembling a package.