The following snippet have to be added to the config.xml under the frontend (for FE-events) – or the adminhtml (for BE-Events) – tag:

<events>
    <event_name>
        <observers>
            <observer_unique_config_name>
                <!-- singleton or model -->
                <type>singleton</type>
                <class>Namespace_Modul_Model_Modelname</class>
                <method>functionToCallOnModelClassInstance</method>
            </observer_unique_config_name>
        </observers>
    </event_name>
</events>