Hi Sam,
I can see that you need help on following two points:
1) Setting up punchout catalog
2) Converting SAP std XML into cXML
Point-1: in SRM system you can follow the path:
SPRO> SAP SRM> SRM Server> Mast data> Content Management > Define external web services
Here you can define a new web service for your catalog (by copying any exiting service). Main parameters which you would need from catalog vendor are:
1) Their OCI complaint catalog URL
2) UserID
3) Password
Use standard call structure and maintain the parameters in the following order.
Also maintain the business partner of the vendor in the details of the web service.
Point-2: SAP SRM generates SAP standard XML format which you can see in SXMB_MONI transaction. This needs to be done in PI where customisation is needed to map standard XML fields to cXML. Then PI will send it to the vendor site. Also, the same mapping needs to be done if there is any PO change message to be sent to the vendor or PO response message which comes from the vendor.
Hope this clarifies and gives you enough insight.
Regards,
Ravi Pachauri