Featherlight gallery within a blog item

I realize this has been asked already a few times, but I also having problems getting the Creating a simple gallery recipe to work in a TwentyFifteen child theme.

My intention is to add a standard Featherlight gallery as a module of a blog item.

My template files:

- templates
-- modular
--- gallery.html.twig
-- partials
--- blog.html.twig
--- blog.item.html.twig


    {% for image in page.media.images %}
        <div class="image-surround">
            {{ image.cropResize(300,200).html }}
        <div class="image-info">
            <h2>{{ image.meta.title }}</h2>
            <p>{{ image.meta.description }}</p>
    {% endfor %}

Content files:

- 02.blog
-- blog item
--- _gallery
---- a.jpg
---- a.jpg.yaml

I think I may be missing something about adding a module to a standard blog item page (thus making it modular), but not sure what?

Does my “_gallery” folder also require a “gallery.md” file? If yes, what should it contain?