Bookkeeping
Say which months you will close, and which receipts you will not hunt.
Bookkeepers lose weeks to clients who think 'catch up the year' includes finding paperwork that was never kept. A useful proposal states the months, the system, and the point at which missing paper becomes the client's problem rather than yours.
Months, system, and the paper they do not have
Write the start month and the end month. Write the tool the books will live in, whether that is the one they already pay for or the one you will move them to. Write who codes the transactions after you leave. A proposal that says 'get the books in order' can be read as last year, the year before, and the shoebox in the office.
Be plain about source documents. If they cannot produce bank feeds and sales invoices, the engagement waits. Chasing a restaurant's cash slips from April is not bookkeeping. It is a second job, and it should be priced as one or refused.
What catch-up does not include
Tax returns, payroll filings, and management accounts are different products. If this proposal is the catch-up and a monthly close, say how many months of the monthly close are inside the fee. If the annual accounts go to their accountant, say that the deliverable is a file the accountant can use, not the filed return.
Name the deadline that matters to them, usually a VAT date or the accountant's annual pack. A bookkeeping timeline that ignores that date will be judged late even if you did what you wrote.