There are several thing I don’t understand yet. I have asked on ActivityPub Plugin - #418 by icaria36 - Plugin - Discourse Meta.
I will document the instructions somewhere when I understand all this better. In the meantime:
To federate a topic, the federated-topics tag must be added. For now, and while we learn how this works, only mods can add this tag.
In the page for this tag, there is a "Federation " tab that shows the information to follow this ActivityPub actor (aka “Mastodon account”). It also shows followers and following.