Steps to add a new host

Generate an SSH host key to be used for secrets ssh-keygen -t ed25519 -f /tmp/ssh_host_ed25519_key -C ""
Go to borgmatic.com, add the generated SSH key and create a new repository
Add a new host entry to nix-secrets/secrets.nix
Generate a repository passphrase in nix-secrets
Commit nix-secrets and run nix flake update in nix-config
Add a README.md and default.nix suited to the host
Define (or generate with nixos-generate-config) a hardware-configuration.nix
Define the disk layout to be used by disko
Commit nix-config
Boot the NixOS installer
Copy the generated SSH host key to /etc/ssh
Run nix run github:nix-community/nixos-anywhere -- --flake .#<hostname> root@<ip address>

Post install

The backup and Tailscale modules won't work until the following steps are completed.

Run sudo borgmatic init --encryption repokey-blake2
Restart borgmatic
Run sudo tailscale up --login-server https://headscale.vimium.net
Visit the URL, then SSH onto vps1 and run headscale --user mesh nodes register --key <key>
(Optionally) Give the Tailscale node a friendly name with headscale node rename -i <index> <hostname>