How do I put an article in a collection, or take it out?

On the article’s form, tick its collections under "Appears in". An article can be in more than one of your collections, and only in your own. In a collection ordered by hand, "Position" sets its place — lower numbers come first. Like any edit to a published article, the change takes effect once it is cleared.

An article you add opens to every live subscriber at once, and they are emailed about it. An article you take out is no longer offered to new subscribers; everyone whose access period was already running keeps reading it until that period ends.