If you're talking about getting Substack publications into your RSS reader, you can add /feed to the end of the relevant URL and then add that into your reader (e.g. https://sinocism.com/feed).
For adding non-Substack publications into the Substack app via RSS, go to reader.substack.com (make sure you're logged in) and click on "Add RSS feed" in the left sidebar.
For adding non-Substack publications into the Substack app via RSS, go to reader.substack.com (make sure you're logged in) and click on "Add RSS feed" in the left sidebar.