how to add a node into an xml

I am not very experienced in Xml and I am wondering what is the easiest way to append a new node an XML.

here is my xml, and I want to append the 1 node to xml.

<?xml version="1.0" encoding="ISO-8859-9"?>
   <CQPN_ROLLS>
      <CHQPN_ROLL DBOP="INS" >
          .....
          .....             
          .....
          .....
          <PAYMENT_LIST>
             ....
             ....
             ....
             <SIGN>1</SIGN>

How am I supposed to find the PAYMENT_LIST node and append to it?