That's a lot of work to do.
TO get it down to bare bones you'll need to:
1. turn off all the community files if you have them.
2. in the editor create a new "witcher" source that you can check when you create a new hero.
3. Read
this thread about restricting what items to see that don't have your "witcher" setting.
4. Cry a little about the amount of work you volunteered to take on.
5. Slog through a lot of work to bring new classes and subclasses that *must* have your new setting attached to them in order to work.
Keep in mind some of the mechanics like called shots and other stuff in the article you linked to will *not* be easy to implement as the 5e SRD mechanics are baked into the program, any rules changes will be nearly impossible to reflect in HL without going to the authoring kit and creating your own rulesets - which is too advanced for me to begin explaining.