Setting up a purchase order system is mostly decisions rather than software, and the decisions are few. Organisations that start with a product evaluation usually spend months configuring something before they have agreed what the rules are. Four decisions, taken first, make the software choice easy and sometimes make it unnecessary for a while.
Decide the threshold and the scope
Which purchases need an order, and above what value. Some categories are impractical to pre-order, and pretending otherwise creates a rule people break daily. Write down the exceptions explicitly. A short, honest rule that people follow controls more spend than a comprehensive one they route around.
Name the approvers and their limits
Take them from your existing delegation of authority rather than inventing a parallel scheme. Name a deputy for each, because absence is the main source of delay. Publish the thresholds so requesters know in advance what their request will need, which removes surprise and reduces order splitting.
Fix numbering and receipts, then tell suppliers
Automatic, unique, never reused numbers, and a named owner for recording what arrives. Then tell suppliers to quote the order number on every invoice and say what happens if they do not. That last conversation is the one that makes the whole thing work, and it takes a quarter of consistent, polite repetition.
Questions people ask about how to set up a purchase order system
Can we start without software?
Yes, and many organisations should. A numbered sequence, an approval email and a receipts log will prove the rules before you pay for anything.
What do we do about invoices with no order?
Decide in advance: return them, or route them to a manual approval path. Absorbing them silently teaches everybody the rule is optional.
How long does it take to bed in?
Expect a quarter or two, because it depends on suppliers and colleagues changing habits rather than on any configuration.