Creating comments via a related section I've been reading on here about suggestions on implementing commenting systems. I'm aware of disqus integration but I want/need  built in commenting.

I have seen it mentioned a couple of times that one approach would be to create a section (presumably a channel) dedicated to comments and the relate that to an entry via an entries field.

I just can't quite wrap my head around how that works out. Do the entries have to be manually selected in order to relate them?

If anyone's built something like this then I would love to know how they tackled it! 

One thing to add is that only registered members will be commenting. And there would be several different sections needing comments.

Thanks!
One approach would be to create a comments channel with an entries fieldtype (with sources checked for any channels that you want to allow commenting; limit set to 1) called 'relatedEntry' (or whatever you like). On your article template, include a for loop to display existing comments relateTo the entry. Check to see if there is a current user, and if yes, include a comments form. In the comments form place a hidden field with the id for the current entry. Something like this:

&lt;article&gt;
  {{ entry.title }}
  {{ entry.body }}
&lt;/article&gt;

&lt;ul class="comments"&gt;
{% for comment in craft.entries.section('comments').relatedTo(entry) %}
   &lt;li&gt;{{ comment.author.fullName }}&lt;br/&gt;{{ comment.body }}&lt;/li&gt;
{% endfor %}
&lt;/ul&gt;

{% if currentuser %}
&lt;form method="post" action="" accept-charset="UTF-8"&gt;
    &lt;input type="hidden" name="action" value="entries/saveEntry"&gt;
    &lt;input type="hidden" name="sectionId" value="2"&gt;
    &lt;input type="hidden" name="redirect" value="{{ entry.url }}"&gt;
    &lt;input type="hidden" name="fields[relatedEntry][]" value="{{ entry.id }}"&gt;

    &lt;label for="comment"&gt;Comment&lt;/label&gt;&lt;br/&gt;
    &lt;input type="text" name="comment"&gt;
&lt;/form&gt;
{% endif %}


name="fields[relatedEntry][] is formated this way because related fields expect an array even if it's only a single related entry.

You could also create threaded comments by creating another related entries fieldtype called parentComment (or whatever you like) and including that as a hidden field in the form as well (you might need ajax or javascript here to help set the hidden field value from the 'comment' link).

To make it a little nicer you could also load the comments form via ajax or reveal it with javascript.

Untested, but hopefully this will point you in the right direction.