If you have ever needed to rewrite/overload a core magento Model.
Its actually pretty easy
Step 1: create the module instantiation file
app/etc/modules/Russellalbin_Sales.xml
<?xml version="1.0"?>
<config>
<modules>
<Indiebooker_Sales>
<active>true</active>
<codePool>local</codePool>
</Indiebooker_Sales>
</modules>
</config>