Start with your franchise agreement
The transfer section of your franchise agreement typically explains what the franchisor needs before approving a sale: an application from the buyer, financial qualifications, training, a new or amended franchise agreement, a release, and a transfer fee. It may also require remodeling or upgrades before or after the transfer, and may give the franchisor the right to buy the business itself on the same terms.
The franchisor will also usually have its own process for the buyer, often including its Franchise Disclosure Document. Ask the franchisor early what it needs and how long approval usually takes, so the purchase agreement timeline is realistic.