Here are my working notes on getting a system up and running.
WARNING: You can run into a hidden problem that will prevent a correct partition setup and
/etc/nixos/configuration.nix from working: if you are setting up a UEFI system, then you need to make sure you boot into the NixOS installation from the UEFI partition of the bootable media. You may have to enter your BIOS boot selection menu to verify this. For example, if you setup a NixOS installer image on a flash drive, your BIOS menu may display several boot options from that flash drive: choose the one explicitly labeled with “UEFI”.
I used these resources: