Creating a helper

Helpers should extend from Mage_Core_Helper_Abstract:

# File: app/code/local/Vendor/Package/Helper/Data.php
class Vendor_Package_Helper_Data extends Mage_Core_Helper_Abstract
    public function multiply($a, $b)
        return $a * $b;

To be able to access is via Mage::helper you need to define a helper alias in a config.xml file to allow the Magento autoloader to find your class:

<!-- File: app/code/local/Vendor/Package/etc/config.xml -->

Assuming your module is correctly configured and you have cleared your cache, you should now be able to use your helper like so:

$result = Mage::helper('alias_here')->multiply(2, 4); // int(8)

Note: if you're using a Data class, its helper name is implied if you don't specify one. For example, the following two examples are identical: