https://rancher.com/docs/os/v1.x/en/installation/running-rancheros/server/install-to-disk/
cloud-config.yml highly recommended to configure the OS; I uploaded mine to /usr/local/www on my pfSense system and specified the url for the ros install to use.
As usual, eth0 is my physical interface, but there are other NICs in the system, so I set all interfaces to no dhcp, then specify a static IP for eth0. Finally, I put in an SSH key so that I can get into the system afterwards.