Purchase order procedures tend to be either absent or twenty pages, and both fail the same test: nobody consults them. Five decisions written on one page cover what people actually need to know, and writing them is also how organisations discover that two of the five have never been decided by anybody.
Threshold and approvers
Which purchases need an order and above what value, with explicit exceptions for categories where pre-ordering is impractical. Then who approves what, taken from your existing delegated authority, with a named deputy for each. Publishing the thresholds is part of this: a requester who knows in advance plans, and one who finds out afterwards splits orders.
Numbering and receipts
Numbers issued automatically from one source, never reused. And who records what arrived, where, and when, with the expectation that it happens at the delivery point rather than being batched into finance later. This second one is the item most often missing and the one everything downstream depends on.
What happens to invoices with no reference
Returned to the supplier, or routed to a manual approval path. Either is defensible; having no stated answer is not, because it means the decision is made differently by whoever handles each one. Write it down, tell suppliers in advance, and be consistent about it.
Questions people ask about purchase order procedure
How long should the procedure be?
One page. Longer procedures are not read and not maintained, and an out-of-date one discredits the parts that are still right.
Who owns it?
Purchasing, with payables consulted, since payables experiences the consequences of every decision in it.
How often should it be reviewed?
Annually, and whenever the exception mix suggests something is not working.