and to follow up on my reply as well...
XML is a very non-specific language... like as its all different, there is specific layout....
for instance, maybe you have heard the terms "SOAP" and "RSS" which are two XML "formats" that have to be in a specific layout....
you'll need to get an example ASX file and see that it has a <thisnode> here, and a child node of <thatnode>, etc....
i hope that makes sense... but the links provided above show you how to create an XML from scratch, its your job to investigate and figure out what the XML file needs to look like to be used as a valid ASX