how to insert data in xml in perticular way using form ?

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C#

XML

ASP.NET4

i want to insert data like

<?xml version="1.0" encoding="UTF-8" ?> 
- <markers>

  <marker lat="23.049089007186403" lng="72.65069425106049" html="Sanjay Prajapati<br>Nishkalank Park,<br>Sardar Chowk,<br>Ahmedabad-382655" label="Sanjay Prajapati" /> 
  <marker lat="23.049589005186403" lng="72.65569421106049" html="Mahesh Prajapati<br>3-Nishkalank Park,<br>Sardar Chowk,<br>Ahmedabad-3825654" label="Mahesh Prajapati" /> 
  <marker lat="23.049989005186403" lng="72.65589421106049" html="Darshak Patel<br>Krishna Park Soc.<br>Sardar Chowk,<br>Ahmedabad-382221" label="Darshak Patel" /> 
  <marker lat="23.052989005186403" lng="72.65989421106049" html="Mitesh Khatra<br>Krishna Park Soc.<br>Sardar Chowk,<br>Ahmedabad-385454" label="Mitesh Khatra" /> 
  </markers>

now new entry should be

<marker lat="from FORM" lng="from FORM" html="from FORM<br>from FORM<br>from FORM<br>from FORM" label="from FORM" />

from FORM is entered data in a form .......
using asp.net c#

give me a proper solution....