It depends a little bit which route you want to take to create the report. It can be created through the Odoo GUI or through a custom module with XML (more advanced).

To create via the GUI, you must enable Developer Settings, then navigate to Settings > Reports. It may be easiest to copy the existing Sales Order report and make the modifications as needed.

To create via XML, you can find details on the \Odoo Reports Documentation.

To add custom report in E-Mail you can try below solution :

 -> Create Custom Module in which you need to override specific "mail template"

 and set your custom report template in "report_template".

 -> If you will not get result, then manually delete this report template and update your new custom module again.(sometimes in odoo records are not updated)