Most accounts payable procedures are written to describe the normal path and are consulted when something abnormal happens, which is why they so often disappoint at the moment of use. A document that gives the clean path a paragraph and the exceptions a page each is inverted relative to the usual one, and it is the version people actually reach for.
The clean path, briefly
Receive, validate, match, route for approval, code, schedule, pay, remit. A paragraph, with the tolerance and the routing rule stated. Anybody doing the job daily knows it, and the document's job is to be right rather than to be thorough about the part everybody already knows.
Each exception, properly
No purchase order, price variance, quantity variance, missing receipt, unrecognised supplier, suspected duplicate, dispute, credit note. For each: how to recognise it, who owns resolving it, what to record, and how long it should take. This is what a new joiner needs and what somebody covering during absence reaches for at four in the afternoon.
The controls, stated plainly
Who may create or amend a supplier record, how bank detail changes are verified, who may release a payment run, and what may be overridden and by whom. These are the parts an auditor asks about, and writing them down is also how you discover that two of them have never actually been decided.
Questions people ask about accounts payable procedures
How long should the document be?
Two or three pages. Long procedures are not maintained, and an unmaintained procedure loses credibility across all of it, including the parts still correct.
Who should write it?
The people doing the work, reviewed by whoever owns the control. Otherwise it describes an intention rather than a practice.
How often should it be reviewed?
When something changes, and annually regardless. A drifted document is worse than none.