First, make sure you've enabled the feeds:
Admin -> News and Newsletters -> Settings
Check the box next to "Enable XML/RSS News".
The "Maximum post length" value determines how many characters of a post are output in the feeds. The default value (255) is fine for now
Now, on to the fun stuff.
All of the XML content is accessed via the ".xml" action...
Other formats can be achieved by adding one of the following to the URL:
;type=atom - Atom
;type=rdf - RDF
;type=rss - RSS 0.91
;type=rss2 - RSS 2.0
The default feed is the 5 most recent posts:
http://www.sggu.com/smf/index.php?action=.xml
There are several "sub-actions" available to this action, specifying exactly what to display:
Recent Posts (displays the most recent posts that you can see)
"Sub-action": recent
Default: Shows 5 most recent posts
Available options:
limit=x - display the "x" most recent posts (if number is less than 5, it will display 5. If number is larger than 255, it will display 255).
board=y - display only the recent posts from board "y"
boards=x,y,z - display only the recent posts from the specified boards
c=x or c=x,y,z - display only the recent posts from boards in the specified category/categories
Example: http://www.sggu.com/smf/index.php?action=.xml;sa=recent;board=72;limit=10
News (displays the first post from the most recent topics that you can see)
"Sub-action": news
Available options:
limit=x - display only "x" items
board=y - display only news topics from board "y"
boards=x,y,z - display only news topics from the specified boards
c=x or c=x,y,z - display only news topics from boards in the specified category/categories
Example: http://www.sggu.com/smf/index.php?action=.xml;sa=news;board=72;limit=10