XML Stack Overflow

I'm looking to create an rss feed for a website using C#. What I'm trying to approach is, once I clicked a button ("Add Feed"), it suppose to create and a Node Element, like this:

<item>
 <title> some title...here </title>
 <link> link to the article ...here </link>
 <description> article's description ...here </description>
</item>

This has to be a child of the <channel> element. So, so far with the code I wrote, it inserts the elements described above in the rss xml file, but it does it outside the <channel> element, like so:

<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0">
  <channel>
    <title>Example Home Page</title>
    <link>http://ift.tt/1IW9t0J;
    <description>Educational Website...</description>
    <image>
      <url>http://ift.tt/1tM4bkT;
      <title>Example.com</title>
      <link>http://ift.tt/1IW9t0J;
    </image>
    <category>Photography</category>
    <language>en-us</language>
  </channel>
<item>
 <title> some title...here </title>
 <link> link to the article ...here </link>
 <description> article's description ...here </description>
</item>
</rss>

This is the code :

 XmlDocument doc = new XmlDocument();

  XmlNode item = doc.CreateElement("item");

  XmlNode Title = doc.CreateElement("title");
  Title.InnerText = TextBoxTitle.Text;
  item.AppendChild(Title);

  XmlNode link = doc.CreateElement("link");
  link.InnerText = "http://www.example.com/" + DropDownListCategory.SelectedItem.Text + ".aspx?key=" + TextBoxLink.Text + ".txt";
  item.AppendChild(link);

  XmlNode description = doc.CreateElement("description");
  description.InnerText = TextBoxDescription.Text;
  item.AppendChild(description);

  doc.DocumentElement.AppendChild(item);
  doc.Save(Server.MapPath("~/rss.xml"));

How can I do that ? Any help or feedback will be appreciated. Thank you !!!