Hi,
You can inherit the template report_delivery_document for customizing the delivery slip.
For eg:

<template id="report_delivery_document_inherit" inherit_id="stock.report_delivery_document">
 <--xpath and add required fields-->
</template>

Regards

Hi,

You have to inherit the corresponding report template and make necessary changes, if you are using odoo enterprise you can do it using the studio, else, you can create a module which inherits this template and make required changes.

See: How To Inherit And Modify Existing PDF Reports in Odoo

Thanks