My NixOS config, quite fresh so not that fancy.
You'll need the bootloader, networking, and system.stateVersion parts from your original config to put in installation.nix.
sudo cp installation.example.nix installation.nix
sudo nano installation.nix
sudo ln -s "$(pwd)/installation.nix" /etc/nixos/installation.nix
sudo rm /etc/nixos/configuration.nix
sudo ln -s "$(pwd)/configuration.nix" /etc/nixos/configuration.nix
sudo nixos-rebuild switch