These are instructions for booting from an Ubuntu liveCD and installing NixOS on a machine. I needed to do this because the NixOS liveCD doesn't work on my machine (NixOS/nixpkgs#5829), so I'm just using the Ubuntu installation media as something to boot into.
Much of this is from discussion at NixOS/nixpkgs#14680.
Get the Ubuntu ISO: http://releases.ubuntu.com/16.04.1/ubuntu-16.04.1-desktop-amd64.iso
Write it to a USB drive with unetbootin