In this tutorial, i will explain how to override magento models using an XML- based configuration file. Models play an important role as they connect our application to database & implement most of the business logic.
we should not change any core files to implement custom features because it is a bad programming if core files are modified. So, what we will do is, create a new module called My_Pant and then we will perform our task to override.

Lets start by creating the following folders to create custom module (Pant):
app/code/local/My/Pant/etc
app/code/local/My/Pant/Model

Create config.xml file of custom module.
Path: "app/code/local/Namespace/Modulename/etc/config.xml" and paste the following contents in that file.

//Our namespace is My & modulename  is Pant

<config>
  <global>

 <models>

       <checkout>

        <rewrite>

             <cart>My_Pant_Model_Cart</cart>

        </rewrite>

       </checkout>

 </models>

 </global>

</config>

Now, create a Model file "app/code/local/My/Pant/Model/Cart.php" and paste the following contents in that file.

class My_Pant_Model_Cart extends Mage_Checkout_Model_Cart

{

// override code in new class

}