Navigate to the side navigation bar and choose "Customers" from customers.

Select the particular customer you wish to add Billing Template.

Scroll down the right-hand window to find the "Billing & Contract" tab and click on it.

Choose the "Billing & Invoice" tab from the provided tabs.

Set "To" and "CC" for invoice contacts. You can choose to set the Primary contact, Secondary contact, or any other customer users and resources as defaults.