As @kerberos said, you can take any tutorial for Nextcloud 20 and just use the latest version of Nextloud instead of Nextcloud 20.
Apart from that, Nextcloud 21 runs just fine with PHP-7.4 and you can always update to PHP-8 later. I also get the feeling that Nextcloud 21 does not yet run as smoothly with PHP-8 as Nextcloud 20 does with PHP-7.4. I have various error messages in the LOG of my test instance with PHP-8. But to be honest, I also have not really looked into it yet. From my experience, however, it is often better if you do not run everything on the “bleeding edge” on a productive Nextcloud instance. It usually takes some time when new versions get released until everything runs smoothly. That does not mean that you can’t use Nextcloud 21 and PHP-8. You just have to be prepared, that you maybe will face some minor issues for which no ready-made solution is available yet.