Most of the gaps in the #Mastodon self-hosting guide (https://docs.joinmastodon.org/user/run-your-own/) are easy to fill from context. I hit two gotchas while following the recipe:

1) You can't set up nginx before you generate your SSL certificate, because nginx will justifiably complain there's no certificate there. Instead run certbot standalone first: https://github.com/mastodon/documentation/pull/826#issuecomment-827558844

2) nginx will return nothing but 502s until you give it access to the mastodon user folder: https://lond.com.br/2018/07/13/running-a-mastodon-instance-using-archlinux.html#intermission-mastodon-directory-permissions