<?xml version="1.0" encoding="UTF-8"?>
<config>
<modules>
    <Customweb_Saferpay>
        <version>1.0.0</version>
    </Customweb_Saferpay>
</modules>
<global>
    <models>
        <saferpay>
            <class>Customweb_Saferpay_Model</class>
            <resourceModel>saferpay_mysql4</resourceModel>
        </saferpay>
    ...

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <Etagen_SaferpayGuestOrderFix>
            <version>0.1.0</version>
        </Etagen_SaferpayGuestOrderFix>
    </modules>
    <global>
        <models>
            <etagen_saferpayguestorderfix>
                <class>Etagen_SaferpayGuestOrderFix_Model</class>
            </etagen_saferpayguestorderfix>
            <saferpay>
                <rewrite>
                    <method>Etagen_SaferpayGuestOrderFix_Model_Method</method>
                </rewrite>
            </saferpay>
        </models>
    </global>
</config>

<?php
class Etagen_SaferpayGuestOrderFix_Model_Method extends Customweb_Saferpay_Model_Method
{
    public function getPaymentPageLink()
    {
        ...code...
    }
}

<?xml version="1.0" encoding="utf-8"?>
<config>
    <modules>
        <Etagen_SaferpayGuestOrderFix>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Customweb_Saferpay />
            </depends>
        </Etagen_SaferpayGuestOrderFix>
    </modules>
</config>

<?xml version="1.0" encoding="UTF-8"?>
<config>
<modules>
    <Customweb_Saferpay>
        <version>1.0.0</version>
    </Customweb_Saferpay>
</modules>
<global>
    <models>
        <saferpay>
            <class>Customweb_Saferpay_Model</class>
            <resourceModel>saferpay_mysql4</resourceModel>
        </saferpay>
        <saferpay_mysql4>
            <class>Customweb_Saferpay_Model_Mysql4</class>
                <entities>
                    <carddata>
                        <table>saferpay_card_data</table>
                    </carddata>
                </entities>
        </saferpay_mysql4>
    </models>
    <blocks>
        <saferpay><class>Customweb_Saferpay_Block</class></saferpay>
    </blocks>
    <helpers>
        <saferpay>
            <class>Customweb_Saferpay_Helper</class>
        </saferpay>
    </helpers>
    <resources>
        <saferpay_write>
            <connection>
                <use>core_write</use>
            </connection>
        </saferpay_write>
        <saferpay_read>
            <connection>
                <use>core_read</use>
            </connection>
        </saferpay_read>
        <saferpay_setup>
             <setup>
                <module>Customweb_Saferpay</module>
                <class>Customweb_Saferpay_Model_Resource_Setup</class>
             </setup>
        </saferpay_setup>
    </resources>
    <events>
        <core_block_abstract_to_html_before>
            <observers>
                <saferpay_html_before>
                    <type>singleton</type>
                    <class>Customweb_Saferpay_Model_Observer</class>
                    <method>htmlBefore</method>
                </saferpay_html_before>
            </observers>
        </core_block_abstract_to_html_before>
        <payment_info_block_prepare_specific_information>
            <observers>
                <saferpay_payment_info_block>
                    <type>singleton</type>
                    <class>Customweb_Saferpay_Model_Observer</class>
                    <method>paymentInfoBlock</method>
                </saferpay_payment_info_block>
            </observers>
        </payment_info_block_prepare_specific_information>
    </events>
    <sales>
        <order>
            <statuses>
                <canceled_saferpay translate="label">
                    <label>Canceled Saferpay</label>
                </canceled_saferpay>
                <pending_saferpay translate="label">
                    <label>Pending Saferpay</label>
                </pending_saferpay>
            </statuses>
            <states>
                <canceled>
                    <statuses>
                        <canceled_saferpay />
                    </statuses>
                </canceled>
                <pending_payment>
                    <statuses>
                        <pending_saferpay />
                    </statuses>
                </pending_payment>
            </states>
        </order>
    </sales>
</global>

<adminhtml>
    <acl>
        <resources>
            <admin>
                <children>
                    <system>
                        <children>
                            <config>
                                <children>
                                    <saferpay translate="title" module="Saferpay">
                                        <title>Saferpay</title>
                                    </saferpay>
                                </children>
                            </config>
                        </children>
                    </system>
                </children>
            </admin>
        </resources>
    </acl>
</adminhtml>

<frontend>
    <blocks>
        <Saferpay>
            <class>Customweb_Saferpay_Block</class>
        </Saferpay>
    </blocks>
    <layout>
        <updates>
            <saferpay>
                <file>saferpay.xml</file>
            </saferpay>
        </updates>
    </layout>
    <routers>
        <Saferpay>
            <use>standard</use>
            <args>
                <module>Customweb_Saferpay</module>
                <frontName>Saferpay</frontName>
            </args>
        </Saferpay>
    </routers>

    <translate>
        <modules>
            <Customweb_Saferpay>
                <files>
                    <default>Customweb_Saferpay.csv</default>
                </files>
            </Customweb_Saferpay>
        </modules>
    </translate>
</frontend>

<default>
    <saferpay>
        <general>
            <mode>1</mode>
            <create_pay_init_url>https://www.saferpay.com/hosting/CreatePayInit.asp</create_pay_init_url>
            <verify_pay_confirm_url>https://www.saferpay.com/hosting/verifypayconfirm.asp</verify_pay_confirm_url>
            <verify_enrollment_url>https://www.saferpay.com/hosting/VerifyEnrollment.asp</verify_enrollment_url>
            <execute_url>https://www.saferpay.com/hosting/Execute.asp</execute_url>
            <pay_complete_url>https://www.saferpay.com/hosting/PayComplete.asp</pay_complete_url>
            <test_account_id>99867-94913159</test_account_id>
            <test_password>XAjc3Kna</test_password>
            <cardref_count>0</cardref_count>
            <cardref_id_scheme>cardref_{ID}</cardref_id_scheme>
            <order_id_schema>order_{ID}</order_id_schema>
            <invoice_settlement>manual</invoice_settlement>
            <capture_invoice>manual</capture_invoice>
            <liability_shift_only>1</liability_shift_only>
            <can_use_scd>0</can_use_scd>
            <safe_cc_data>0</safe_cc_data>
            <usernotify>0</usernotify>
            <notifyaddress>0</notifyaddress>
        </general>
        <paymentpage>
            <autoclose>0</autoclose>
            <ccname>1</ccname>
            <showlanguages>1</showlanguages>
        </paymentpage>
    </saferpay>
</default>

<models>
        <saferpay>
            <class>Customweb_Saferpay_Model</class>
            <resourceModel>saferpay_mysql4</resourceModel>
        </saferpay>
    </models>

<models>
        <etagen_saferpayguestorderfix>
            <class>Etagen_SaferpayGuestOrderFix_Model</class>
        </etagen_saferpayguestorderfix>
        <saferpay>
            <rewrite>
                <method>Etagen_SaferpayGuestOrderFix_Model_Method</method>
            </rewrite>
        </saferpay>
    </models>

class Customweb_Saferpay_Model_Method
{
    public function getPaymentPageLink()
    {
        return get_class($this);
    }

}

class Etagen_SaferpayGuestOrderFix_Model_Method extends  Customweb_Saferpay_Model_Method{

}

$model = Mage::getModel('saferpay/method');
echo $model->getPaymentPageLink();

<models>
        <etagen_saferpayguestorderfix>
            <class>Etagen_SaferpayGuestOrderFix_Model</class>
        </etagen_saferpayguestorderfix>
        <saferpay>
            <deprecatedNode>etagen_saferpay</deprecatedNode>
        </saferpay>
        <etagen_saferpay>
            <rewrite>
                <method>Etagen_SaferpayGuestOrderFix_Model_Method</method>
            </rewrite>
        </etagen_saferpay>
    </models>

Mage::getModel('customer1/customer2')
                  |           |_ Is the <customer2> tag inside the <rewrite> element.
                  |_ Is the <customer1> tag outside the <rewrite> element.

<global>
        <models>
            <customer1>
                <rewrite>
                    <customer2>Super_Awesome_Model_Customer</customer2>
                </rewrite>
            </customer1>
        </models>
  </global>

class Super_Awesome_Model_Customer extends Mage_Customer_Model_Customer