It took me a while to do so, as I found it not too trivial until I just
literally copy-pasted the template code to `footer.html` and saving it
as `footer.html.in`, which openring itself takes as input, and outputs
the former file already populated, more or less.

This is what I have just before this blog's footer. 
(angle brackets replaced with < and > for presentation)

```
<pre> <=^.^=> Articles from blogs I follow </pre>
<section class="webring">
	<section class="articles">
		{{range .Articles>
		<div class="article">
			<h4 class="title"><a href="{{.Link}}" target="_blank" rel="noopener">{{.Title}}</a></h4>
			<p class="summary">{{.Summary}}</p>
			<small class="source">via <a href="{{.SourceLink}}">{{.SourceTitle}}</a></small>
			<small class="date">{{.Date | datef "January 2, 2006"}}</small>
		</div>
		{{end}}
	</section>
	<p class="attribution">Generated by <a href="https://git.sr.ht/~sircmpwn/openring">openring</a></p>
</section>
```

And the CSS, which I copied verbatim into main.css, except for the background color
```
.webring .articles { display: flex; flex-wrap: wrap; margin: -0.5rem; }
.webring .title { margin: 0; }
.webring .article { flex: 1 1 0; display: flex; flex-direction: column; margin: 0.5rem; padding: 0.5rem; background: #30; min-width: 10rem; }
.webring .summary { font-size: 0.8rem; flex: 1 1 0; }
.webring .attribution { text-align: right; font-size: 0.8rem; color: #555; }
```

Thing is, I wasn't able to integrate with it the Makefile + conf.mk
combo I use for generating the output, so I have to run it before in
order to get the footer.html and then build as normal. Beyond that it
works fine.