Guides
The paperwork, explained properly.
Written for people doing the job, not for search engines. If a guide doesn't tell you something you can act on, it shouldn't be here.
What goes on a purchase order, and why each part matters
A PO is an offer to buy on stated terms. Getting the parts right is what makes it enforceable and what makes the invoice match.
Numbering purchase orders so accounts can actually match them
Duplicate and re-used PO numbers are the most common cause of an invoice sitting unpaid in a query pile.