These posts are really useful to get a grasp of ActivityPub (if you have a programming background): https://rknight.me/building-an-activitypub-server/ https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/

And of course the official spec (although its less useful): https://www.w3.org/TR/activitypub/