Build me a purchase order system

We are a small medium charity in NW London and focus on life-long learning, youth activities and community development. We have been pitching in during the COVID pandemic to support local residents. We have grown as a charity over the last few years and have lots of areas of development on process. The request here is for someone to design a simple purchase order approval process. I have been exploring this with Microsoft Forms and flows but need some help. We use office 365 but you would advise best what to use. We also use Quickbooks online.