Hi!

While I haven't used it in a few months, here is what I did to set it
up:

--8<---------------cut here---------------start------------->8---
(operating-system
  (inherit %bababa/os)
  (packages (cons* nix
                   ...))
  (services (cons*
             (service nix-service-type)
             ...
             ))
  )
--8<---------------cut here---------------end--------------->8---

Then, as per the manual:

--8<---------------cut here---------------start------------->8---
        • Add a Nix channel and update it.  See Nix Package Manager
          Guide (https://nixos.org/nix/manual/).

        • Create a symlink to your profile and activate Nix profile:

          $ ln -s "/nix/var/nix/profiles/per-user/$USER/profile" ~/.nix-profile
          $ source /run/current-system/profile/etc/profile.d/nix.sh
--8<---------------cut here---------------end--------------->8---

Hope that helps!

-- 
Pierre Neidhardt
https://ambrevar.xyz/