Struggle with featherlight gallery

I am trying to get a gallery similar to the demo page ( with the grav plugin. In particular browsing through the images.
I can display the thumbs and have a lightbox open when clicking, but I would like to be able to scroll through the images (next, previous). I don’t know how to do that. The original documentation is too vague for me. and hard to translate to Grav, as I am just learning it.

My gallery.twig.html file:
{% extends ‘partials/base.html.twig’ %}

{% block content %}


{% endblock %} '''