Basically all you do is check for the presence of an item number field in the url query string. If it's present, just show that item. Here is some demo code:

<?php
 
require 'simplepie.inc';
 
$url = 'http://news.google.com/news?ned=us&topic=h&output=rss';
 
$feed = new SimplePie();
$feed->set_feed_url($url);
$feed->init();
 
// default starting item
$start = 0;
 
// default number of items to display. 0 = all
$length = 0; 
 
// if single item, set start to item number and length to 1
if(isset($_GET['item']))
{
        $start = $_GET['item'];
        $length = 1;
}
 
// set item link to script uri
$link = $_SERVER['REQUEST_URI'];
 
// loop through items
foreach($feed->get_items($start,$length) as $key=>$item)
{
 
        // set query string to item number
        $queryString = '?item=' . $key;
 
        // if we're displaying a single item, set item link to itself and set query string to nothing
        if(isset($_GET['item']))
        {
                $link = $item->get_link();
                $queryString = '';        
        }
 
        // display item title and date    
        echo '<a href="' . $link . $queryString . '">' . $item->get_title() . '</a>';
        echo ' <small>'.$item->get_date().'</small><br>';
 
        // if single item, display content
        if(isset($_GET['item']))
        {
                echo ' <small>'.$item->get_content().'</small><br>';
        }
        echo '<br>';
}
 
?>