You can make the USB drive by downloading the image off of nixos.org and then use Etcher, dd, or your favorite iso to USB drive utility.

The minimum version of NixOS does not come with a GUI installer.

https://nixos.org/manual/nixos/stable/#ch-installation

The manual contains all the info needed. For a minimum install, there are a couple of steps that you need to perform, before you can install.

1. Format hard disk
2. Create config file
3. Install

Format Hard Disk

We’ll assume that /dev/sda is our target disk. This will overwrite the disk. Make sure you don’t need anything on it.

parted /dev/sda -- mklabel gpt
parted /dev/sda -- mkpart root ext4 512MB -8GB
parted /dev/sda -- mkpart swap linux-swap -8GB 100%
parted /dev/sda -- mkpart ESP fat32 1MB 512MB
parted /dev/sda -- set 3 esp on

Format the partitions

mkfs.ext4 -L nixos /dev/sda1
mkswap -L swap /dev/sda2
mkfs.fat -F 32 -n boot /dev/sda3
mount /dev/disk/by-label/nixos /mnt
mkdir -p /mnt/boot
mount /dev/disk/by-label/boot /mnt/boot
swapon /dev/sda2

Create a basic config file

nixos-generate-config --root /mnt

You can edit the config to make any changes you need. You may want to uncomment the user lines to setup a new user.

nano /mnt/etc/nixos/configuration.nix

Install NixOS

nixos-install

Last step is to setup the root and user password.

passwd

Reboot the machine once the password is set.

After you log in, set the user password.

passwd username