This is the gist of how to setup a NixOS server on a Hetzner Cloud instance with an admin user, ssh access and configuration management via git.
- Create a Hetzner Cloud instance and click to enter it.
- Stop the instance (top right corner icon).
- Go to ISO Images.
- Search for "nixos" and click mount.

- Start the instance again (top right corner icon).
- Open the console (top right corner icon).
- Get the IP address by executing
ip --brief --color address. The address can also be optained from the Hetzner Cloud web interface.