I have something like this in my overlays.nix:

  self: super:
  
  let
    callPackage = super.callPackage;
  in {
    darktable = callPackage
      ./pkgs/darktable { inherit (super) darktable; };
  }

And my default.nix in pkgs/darktabele looks like this:

  { darktable, fetchurl, ... }:
  
  darktable.overrideAttrs (_: rec {
    version = "3.0.0";
    src = fetchurl {
      url = "https://github.com/darktable-org/darktable/releases/download/release-${version}/darktable-${version}.tar.xz";
      sha256 = "7195a5ff7ee95ab7c5a57e4e84f8c90cc4728b2c917359203c21293ab754c0db";
    };
  })