Ditto is a snippet that is included with your installation of MODx. It’s used for blog and news listings, or anything else that you want a list of summaries and links for.
Create a resource (document), to be the container for your rss items. Now add resources as children to that, one for each row in your feed.
On the parent document, use a Ditto snippet call:
[!Ditto? &format=`rss` &parents=`xx`!]
Where the `xx` is the document ID of the parent resource (you can leave it out, since the Ditto call is on the parent resource).
That’s all. Link to that parent resource for your feed. You can take a look at my site’s feed; log in to my manager as guest - guestuser, then take a look at the "Modx From the Inside" resource (#49) at the bottom of the Resource Tree. It is set to make a feed from my "chapters" of the MODx From the Inside book I’m slowly writing.
If you take a look at the "inside" template (Elements -> Manage Elements, Templates tab) you’ll see the line
<link rel="alternate" type="application/rss+xml" title="MODx From the Inside" href="[(site_url)][~49~]" >
That’s all that is needed to create a feed.
Of course, you can customize the feed if you like...
http://ditto.modxcms.com/tutorials/custom-rss-feed.html