Follow below steps, create one custom module
app/code/local/Custom/Module/etc/config.xml
<?xml version="1.0"?>
<config>
<modules>
<Custom_Module>
<version>0.1.0</version>
</Custom_Module>
</modules>
<admin>
<routers>
<adminhtml>
<args>
<modules>
<Custom_Module before="Mage_Adminhtml">Custom_Module</Custom_Module>
</modules>
</args>
</adminhtml>
</routers>
</admin>
<global>
<helpers>
<module>
<class>Custom_Module_Helper</class>
</module>
<adminhtml>
<rewrite>
<data>Custom_Module_Helper_Adminhtml_Data</data>
</rewrite>
</adminhtml>
</helpers>
<blocks>
<module>
<class>Custom_Module_Block</class>
</module>
<adminhtml>
<rewrite>
<customer_grid>Custom_Module_Block_Adminhtml_Customer_Grid</customer_grid>
</rewrite>
</adminhtml>
</blocks>
</global>
</config>
app/code/local/Custom/Module/Block/Adminhtml/Customer/Grid.php
<?php
class Custom_Module_Block_Adminhtml_Customer_Grid extends Mage_Adminhtml_Block_Customer_Grid {
}
app/code/local/Custom/Module/Helper/Adminhtml/Data.php
<?php
class Custom_Module_Helper_Adminhtml_Data extends Mage_Adminhtml_Helper_Data {
}
app/code/local/Custom/controllers/CustomerController.php
<?php
include_once("Mage/Adminhtml/controllers/CustomerController.php");
class Custom_Module_CustomerController extends Mage_Adminhtml_CustomerController
{
}
?>
And finally declare your module
app/etc/modules/Custom_Module.xml
<?xml version="1.0"?>
<config>
<modules>
<Custom_Module>
<active>true</active>
<codePool>local</codePool>
<version>0.1.0</version>
</Custom_Module>
</modules>
</config>