It's limited to Public blogs right now (a paid feature) so we can slowly roll this out and plug holes as needed -- once I'm confident things are stable in the next few days, it'll be available to everyone for free.
You can read a bit more about the update, and I'd love to hear any feedback / ideas / bugs here! https://discuss.write.as/t/activitypub-support/64
In the web UI of Mastodon, you enter the address of the thing you're trying to follow in the search box, and you will get a result with an option to follow.
And I must say, that's pretty friggin cool. 😮 Thank-you Mastodon, ActivityPub, and write.as!
You'll notice this with new Mastodon accounts too -- Masto just doesn't fetch old posts. But once someone on your instance is following an account, you'll start to see the profile populated with posts
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!