@appleAppy - Dino and mkdunker have created some nice tutorials and write ups here and here on this topic. I'd suggest reviewing those and when you're up and running see the docs page for the JSONtoXML policy here to understand the full set of functionality available.
If you'd like to try out the JSONtoXML policy you can use a 'No Target' proxy to test out the functionality, Dino's screen-cast here covers this. The Apigee UI has changed since then but I'm sure you can follow along.
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