PowerQuery and XML

I've been asked to convert the contents of an XML file in to another XML file in a slightly different format. I've successfully used Get & Transform to import the xml file and sort out the various table fields that are required. I've then used a query and a manually built table to achieve the transformation required so that I have the transformed data as a query on another sheet.
I now need to save that sheet as an XML file but it tells me that I need to define XML Mappings. Please can anyone point me in the direction of a web page or even a book that tells me how to create the XML mappings.

Thanks
Paul Christie