Due diligence agents are not asking whether you are wealthy. They are asking whether they can trace each material sum from origin to the account it will be paid from, without gaps.
Build the file backwards. Start with the account the payment leaves from, then evidence every inflow into it above the materiality threshold, then evidence the origin of those inflows.
Three documents do most of the work: audited or accountant-certified financials for business income, a sale agreement plus completion statement for asset disposals, and bank statements that visibly connect the two.
The single most common failure is a large, round, unexplained transfer between the applicant's own accounts. It is almost always innocent, and it will almost always be queried. Explain it before it is asked about.
Where funds are gifted or inherited, evidence the donor to the same standard as the applicant. A gift letter alone is not a source of funds.