Alright couple things to notice with this.
1.) The booking tables has what's called a Surrogate Key, the autonumber field call BKG_ID. This ensures uniqueness in the records. Really there should probably be other restrictions but we won't worry about those.
2.) The form's recordsource is based on a query (stored in the form) that joins the 2 tables.
3.) New records can be added using the navigation buttons at the bottom.
4.) Before booking a customer, they MUST have a record already in the customer table
Let me know if you have any questions!