Better idea define a new layout file
from you custom module config.xml
file .
<frontend>
<layout>
<updates>
<custommodule>
<file>custommodule.xml</file>
</custommodule>
</updates>
</layout>
</frontend>
Then you need put this custommodule.xml
file at app/design/frontend/YourPackage/YourTemplate/layout/
Now,you can add your template file (.phtml) using
Page handler
We know that checkout cart page handler is checkout_cart_index
and checkout success page is checkout_onepage_success
.
on custommodule.xml.you need put this code:
<?xml version="1.0"?>
<layout version="0.1.0">
<checkout_cart_index>
<reference name="content">
<block type="core/template" name="Giveaname" as="GiveAliasename" template="YourPathlocation" />
</reference>
</checkout_cart_index>
<checkout_onepage_success>
<reference name="content">
<block type="core/template" name="Giveaname" as="GiveAliasename" template="YourPathlocationofTempkay" />
</reference>
</checkout_onepage_success>
</layout>
The advantage of custom layout file
is if module disable then the template is not works.