I assume that latest stable (e.g. 20.03) ISO will be used for installation.
Default network configuration is VPN-only, so if you don't have plans to use it you need to change iptables rules (remove iptables -P OUTPUT DROP from networking.nix) and remove services.openvpn.servers.vpn from networking.nix.
GUI settings is optimized for 3840x2160 on 15".

Installation

parted /dev/vda mklabel gpt
parted /dev/vda mkpart EFI fat32 0% 512M
parted /dev/vda set 1 esp on
parted /dev/vda mkpart NIX ext4 512M 100%

cryptsetup luksFormat /dev/vda2
cryptsetup open /dev/vda2 nix

mkfs.vfat -F32 /dev/vda1
mkfs.ext4 /dev/mapper/nix

mount /dev/mapper/nix /mnt/
mkdir /mnt/boot
mount /dev/vda1 /mnt/boot

nix-env -iA nixos.gitMinimal
git clone https://code.dumpstack.io/infra/localhost.git /mnt/etc/nixos/

cd /mnt/etc/nixos

cp wireless-networks.nix.example wireless-networks.nix
nano wireless-networks.nix

cp secrets.nix.example secrets.nix
nano secrets.nix

nix-channel --add https://nixos.org/channels/nixos-20.09 nixos
nix-channel --add https://nixos.org/channels/nixos-20.09-small nixos-small
nix-channel --update

nixos-generate-config --root /mnt

nixos-install
reboot

After install

Initial password for user is user.

sudo nix-channel --add https://nixos.org/channels/nixos-20.09 nixos
sudo nix-channel --add https://nixos.org/channels/nixos-20.09-small nixos-small
sudo nix-channel --update