nixos-up is a dead-simple install wizard for NixOS. It's the fastest way to get from ISO to working installation.
From the NixOS installation USB/CD:
sudo nix-shell https://github.com/samuela/nixos-up/archive/main.tar.gz
You can check out a video demonstrating the process here: https://youtu.be/f7DzbiRD99Q.
In this directory run servefile --tar --compression gzip --port 12345 .. Then, while that's running nix-shell -p ngrok --run "ngrok http 12345".
Now in your VM/device, run
nix-collect-garbage && sudo nix-shell http://blah-blah-blah.ngrok.io/nixos-up.tar.gz
You may need sudo umount --lazy /mnt periodically as well.