You can find the full instructions here:
… but basically everything is cached by hydra.nixos.org (and should hopefully remain cached), so at the time of this writing you can now run:
$ nix run nixpkgs#darwin.builder
… and that will launch the macOS builder for Linux. You also have to edit your nix.conf to use the builder (see the instructions linked above for more details)
I'll be writing up a blog post soon that will explain the background and motivation in more detail