I also couldn't find a good tutorial, but another commenter here pointed to this blog post: https://blog.joinmastodon.org/2018/06/how-to-implement-a-bas...

Maybe it's a good starting point?

The specification itself is similar in practice (technically, it's JSON-LD and not AS. The latter is a subset of the former):

JSON-LD: https://www.w3.org/ns/activitystreams.jsonld

Human-readable: https://www.w3.org/ns/activitystreams

W3C maintains a list of implementations [1]. Not sure how up to date it is though.

[1] https://github.com/w3c/activitystreams/tree/master/implement...