Hi Sophia,
I would suggest you to take any sample define.xml file (e.g., from CDISC Define-XML 2.0 standard package or PhUSE updated Pilot data), create Excel specs in P21C tool (Import define.xml) and use them as an example.
To add a reference to document you need to create a records in Documents tab. Then use ID value for Document column in other tabs like Method or Comments:
Kind Regards,
Sergiy