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.atom' );
$renderer = new JDocumentRendererAtom( $feedDoc );
echo $renderer->render();

Result

<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="en">
	<title>Feed Title</title>
	<subtitle></subtitle>
	<link rel="alternate" type="text/html" href=""/>
	<id></id>

	<updated>2007-05-26T02:55:54-04:00</updated>
	<author>
		<name>Feed Editor</name>
	</author>
	<generator uri="http://joomla.org" version="1.5">Joomla! 1.5 - Open Source Content Management</generator>
<link rel="self" type="application/atom+xml" href="http://example.com/feed.php" />
	<entry>
		<title>Item Title</title>

		<link rel="alternate" type="text/html" href="http://example.com/index.php?item_title"/>
		<published>2007-05-26T02:55:54-04:00</published>
		<updated>2007-05-26T02:55:54-04:00</updated>
		<id>http://example.com/index.php?item_title</id>
		<author>
			<name>Joe Blow</name>
		</author>

		<summary type="html">About the Item</summary>
	</entry>
</feed>