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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):

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