Magento: Create a new observer

Create a small module to save mata details automatically:
1.app/etc/modules/Sushant_Meta.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Sushant_Meta>
            <codePool>local</codePool>
            <active>true</active>
            <depends>
                <Mage_Catalog />
            </depends>
        </Sushant_Meta>
    </modules>
</config>

2. app/code/local/Sushant/Meta/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Sushant_Meta>
            <version>1.0</version>
        </Sushant_Meta>
    </modules>
    <global>
        <models>
            <sushant_meta>
                <class>Sushant_Meta_Model</class>
            </sushant_meta>
        </models>
    </global>
    <adminhtml>
        <events>
            <catalog_product_save_before><!-- observe the event -->
                <observers>
                    <sushant_meta>
                        <class>Sushant_Meta_Model_Observer</class>
                        <method>autoSaveMetaDescription</method>
                    </sushant_meta>
                </observers>
            </catalog_product_save_before>
        </events>
    </adminhtml>
</config>

3.app/code/local/Sushant/Meta/Model/Observer.php

<?php 
class Sushant_Meta_Model_Observer {
    public function autoSaveMetaDescription($observer) {
        $product = $observer->getEvent()->getProduct();
        $metaTitle = $product->getName();
        $metaDescription = "Buy ". $product->getName()." for Rs ".number_format($product->getFinalPrice(), 2);
        $metaKeyword = $product->getName().",".$product->getShortDescription();
        $product->setMetaDescription($metaDescription)->setMetaTitle($metaTitle)->setMetaKeyword($metaKeyword);
        
    } 
}