The first step is to build a DTD for this structure. Instead of doing it manually you can just go to the query outputting the NRDM structure and select "Generate DTD".
You save the DTD file and generate a DTD object within DI by going to the object library - formats - DTDs and create a new one based on that file.
How many XML strings do we want? One. One containing all order information. So again, we build a NRDM in a Query with Row Generation being the root node (and the only "From") and its child is one schema, our constructed order NRDM.
After that, we place another query where we have one output column, the future XML string, and its mapping is the conversion function "load_into_xml".
The reverse function exists as well in was used in the chapter How to split a comma separated String into multiple rows?.