Punchout Catalog - Are there any Punchout URL guides we can provide our suppliers?

Content

I have set up a number of Punchout catalogs with new suppliers, on occasions the connection works first time, other times the connection error log needs to be shared before we reach a successful test... However I still have some suppliers who are not able to provide with a URL and shared secret that work, and the error logs are not supporting their cause. Are there any clear guides I can provide the supplier at the beginning of the process to enable them to provide me with a URL that works first time?     

Version

Currently using 20D, will shortly upgrade to 21A