To rewrite exiting Magento models, you can add the following snippet to your modules config.xml: <?xml version=”1.0″?> <config> <modules> <AskSheldon_Module> <version>0.1.3</version> </AskSheldon_Module> </modules> <!– … –> <global> <models> <checkout> <rewrite> <session>AskSheldon_Module_Model_Checkout_Session</session> <type_onepage>AskSheldon_Module_Model_Checkout_Type_Onepage</type_onepage> </rewrite> </checkout> </models> </global> <!– … –> </config> In this case Mage_Checkout_Model_Session and Mage_Checkout_Model_Type_Onepage are rewritten.