<config>
<frontend> [1]
<routers>
<checkout> [2]
<args>
<modules>
<yourModule before="Mage_Checkout">Namespace_Module_Overwrite_Checkout</yourModule> [3]
</modules>
</args>
</checkout>
</routers>
</frontend>
</config>
[1] Depending on the controllers area ,”frontend” or “admin”
[2] Router node of the module to be overridden (look it up in the config.xml of that module)
[3] The tag can be anything, it must be unique within the node.
Your_Module_Overwrite_Checkout (is mapped to directory)
Your/Module/controllers/Overwrite/Checkout/