Saving one to many in a form

After finishing Vidly i tried starting my own little project.
A simple invoice program. With customers, products and orders.
I can’t figure out how to save multiple records in one form to the database? I’ve Created the form and added some extra fields but its only saving one record?

Use the concepts on module 10 “Building a Feature End-to-End Systematically”, where mosh teaches how to develop a new feature, a rental for a client with a list of movies…