ELI5: ActivityPub and federation

I'm really excited to jump on the Activitypub/Fediverse train, but I'm still trying to wrap my head around how it works. Kbin is federated (or its supposed to be, apparently DDoS protection is hampering that?), but I'm not sure how I can sign up with my Mastodon account. I already have one, @ArchieBAntlers , and apparently...