You have defined your adminhtml layout xml with name packingflyerreminder.xml

Create a file app/design/adminhtml/default/default/layout/packingflyerreminder.xml with below code

<?xml version="1.0"?>
<layout version="0.1.0">
    <adminhtml_orderpreparation_packing_index>
        <reference name="onepagepreparation">
            <action method="setTemplate"><template>MyOwnFiles/Packing/Index.phtml</template></action>
        </reference>
    </adminhtml_orderpreparation_packing_index>
</layout>

No create your phtml file at app/design/adminhtml/default/default/template/MyOwnFiles/Packing/Index.phtml with your custom code.

You have defined your adminhtml layout xml with name packingflyerreminder.xml

Create a file app/design/adminhtml/default/default/layout/packingflyerreminder.xml with below code

<?xml version="1.0"?>
<layout version="0.1.0">
    <adminhtml_orderpreparation_packing_index>
        <reference name="onepagepreparation">
            <action method="setTemplate"><template>MyOwnFiles/Packing/Index.phtml</template></action>
        </reference>
    </adminhtml_orderpreparation_packing_index>
</layout>

Now create your phtml file at app/design/adminhtml/default/default/template/MyOwnFiles/Packing/Index.phtml with your custom code.