A defined scope of work, the total fee and its payment schedule, what is refundable and at which stage, the adviser's authorization to give immigration advice, and their written statement of the current official terms.
Five items, all before payment
Hypothetical example: a Hampton family are told the fee covers everything and that the details will follow after the retainer is paid. Reverse that order. A scope of work should say what the adviser will actually do and what remains the family's responsibility. The fee statement should include anything payable later and disclose whether any part is a commission. The refund terms should be explicit by stage. The authorization question matters because in most cases only a qualified lawyer or accredited representative may give immigration advice for a fee. And a written statement of the current official terms lets the family verify the adviser against the official page. If any of the five is refused, that refusal is itself the answer.
Identity and family information, a precise description of the actual proposal, and specific questions about eligibility and funding, followed by whatever USCIS forms and evidence the official instructions require after the initial step.