How to install packages from unstable channel via overlays?

I'm trying to install a single package from the unstable channel but don't have the slightest clue about how because the docs aren't really helpful at all. They only show how to create overlays etc. but not how to use them.

This overlay will be made available on your flake’s overlays output with the same name as the directory that you created. is not really an explanation for users who never worked with overlays before. The docs need more examples and not just on how to create the file.

# overlays/sketchybar-app-font/default.nix

{
channels,
namespace,
inputs,
... }:

final: prev: {
    inherit (channels.unstable) sketchybar-app-font;
}

Where do I have to register the overlay?
How do I install the unstable package now in my modules/sketchybar/default.nix?

{ options, config, lib, pkgs, ... }:

with lib;
with lib.shiro;
let
  cfg = config.shiro.tools.sketchybar;
in
{
  options.shiro.tools.sketchybar = with types; {
    enable = mkBoolOpt false "Whether or not to install sketchybar";
  };

  config = mkIf cfg.enable {
    environment.systemPackages = with pkgs; [ sketchybar ];

    fonts.packages = with pkgs; [
        sketchybar-app-font
    ];

    services.sketchybar = {
      enable = true;
      package = pkgs.sketchybar;
    };
  };
}

Start by reading the existing overlay documentation referenced in the issue, including the explanation of the flake's overlays output. Add examples showing where the overlay is registered and how the unstable package is referenced from a module, using the provided overlay and sketchybar module as context. Done means a newcomer can follow the documented steps to install one unstable package.