Example

$feedDoc =& JDocument::getInstance( 'feed' );
$feedDoc->syndicationURL = 'http://example.com/feed.php';
$feedDoc->copyright = 'Copyleft(C)';
$feedDoc->editor = 'Feed Editor';
$feedDoc->title = 'Feed Title';
$feedDoc->subtitle = 'Feed sub title';


$item = new JFeedItem();
$item->title            = 'Item Title';
$item->link             = 'http://example.com/index.php?item_title';
$item->description      = 'About the Item';
$item->date             = mktime();
$item->author           = 'Joe Blow';
$item->category         = 'General Category';

$feedDoc->addItem( $item );

jimport( 'joomla.document.renderer' );
jimport( 'joomla.document.feed.renderer.rss' );
$renderer = new JDocumentRendererRSS( $feedDoc );
echo $renderer->render();

Result

<rss version="2.0">
	<channel>
		<title>Feed Title</title>
		<description></description>
		<link></link>

		<lastBuildDate>Sat, 26 May 2007 03:09:45 -0400</lastBuildDate>
		<generator>Joomla! 1.5 - Open Source Content Management</generator>
		<language>en</language>
		<copyright>Copyleft(C)</copyright>
		<managingEditor>Feed Editor</managingEditor>
		<item>

			<title>Item Title</title>
			<link>http://example.com/index.php?item_title</link>
			<description><![CDATA[About the Item]]></description>
			<author>Joe Blow</author>
			<category>General Category</category>
			<pubDate>Sat, 26 May 2007 03:09:45 -0400</pubDate>

		</item>
	</channel>
</rss>