I have experimented first by copying the app/code/core/Mage/Core/Model/Message folder to app/code/local/Mage/Core/Model/Message then set Abstract::getText() to add a piece of text so I’d know that was the module being executed.
I called it by adding this code to the page template <?=Mage::getSingleton('core/message')->success('message')->getText()?> That works OK.
Then I copied that folder to app/code/core/Mage/Custom/Model/Message and set getText() to add different text.
I also changed the class names to Mage_Custom_Model_...
In app/etc/modules/Mage_All.xml I also added
This does not work, so I created app/code/core/Mage/Custom/etc/config.xml using this code:
However, this produces an Internal Server Error.
I have read through http://www.magentocommerce.com/wiki/how-to/create-payment-method-module and cannot see what else I can change or create.
What do I need to do differently to make the Custom module work?