I don't know how to customize Misago.
Misago is terrible thing to pick up as your first Django app to fiddle with. Its very large. It heavily customizes (or works around) standard Django features.
Misago is also not a "Django app". You can't add it to
INSTALLED_APPS of existing project to add forum to your Django project. You need to run it separately and use other approach (like Django-SSO) to integrate user system over the API.
How to customize models?
You shouldn't do that unless you want to maintain your own version of Misago.
How to customize templates?
Misago uses React.js to render it's HTML. Django templates are here for clients that don't support JS to still be able to consume site's content (eg. search engines).
I think Misago is best in django forum package. But no have tutorial..
Thanks but I'm not thinking about it as "Django package". Furthermore in future versions I'm no longer using Django so... ;)