The default Magento onepage checkout includes six steps for the customer to complete. However, sometimes you may have a requirement to create an extra checkout step. An example of this might be an option for your customer to choose a free gift as part of their order, or an extra step to collect special delivery instructions. Using delivery instructions as an example, we'll demonstrate how this can be achieved.
The first file we need to modify is app/code/core/Mage/Checkout/Block/Onepage.php. Obviously we don't want to modify the code in the core context, so copy the file to app/code/local/Mage/Checkout/Block/Onepage.php. Magento will use this file automatically.