Dispatch event

Suppose I like to add an event on payment save

do not add code in core file add it in overwrite file this is just an example

then i will add this code

magento\app\code\core\Mage\Checkout\controllers\OnepageController.php" 

on savePaymentAction function

 Mage::dispatchEvent('on_payment_methode_save', array('data' => $data));

 public function savePaymentAction()

    {

        if ($this->_expireAjax()) {

            return;

        }

        try {

            if (!$this->getRequest()->isPost()) {

                $this->_ajaxRedirectResponse();

                return;

            }

   

   

            $data = $this->getRequest()->getPost('payment', array());

            $result = $this->getOnepage()->savePayment($data);

    Mage::dispatchEvent('on_payment_methode_save', array('data' => $data)); 

            // get section and redirect data

            $redirectUrl = $this->getOnepage()->getQuote()->getPayment()->getCheckoutRedirectUrl();

            if (empty($result['error']) && !$redirectUrl) {

                $this->loadLayout('checkout_onepage_review');

                $result['goto_section'] = 'review';

                $result['update_section'] = array(

                    'name' => 'review',

                    'html' => $this->_getReviewHtml()

                );

            }

            if ($redirectUrl) {

                $result['redirect'] = $redirectUrl;

            }

        } catch (Mage_Payment_Exception $e) {

            if ($e->getFields()) {

                $result['fields'] = $e->getFields();

            }

            $result['error'] = $e->getMessage();

        } catch (Mage_Core_Exception $e) {

            $result['error'] = $e->getMessage();

        } catch (Exception $e) {

            Mage::logException($e);

            $result['error'] = $this->__('Unable to set Payment Method.');

        }

        $this->getResponse()->setBody(Mage::helper('core')->jsonEncode($result));

    }

now  add an event in your config XML file

<events>

 <on_payment_methode_save>

    <observers>

     <Company_Module_Model_Observer>

      <class>Company_Module_Model_Observer</class>

      <method>onPaymentSave</method>

     </Company_Module_Model_Observer>

    </observers>

    </on_payment_methode_save>

  </events>

and further in your Model/observer.php file

public function orderGridLodBefore(Varien_Event_Observer $observer){

    $event = $observer->getEvent()->getData();

}