So since I have all my CUS_NUMBER's in the Customers table, do I just have to copy and paste them into the Invoices table into the INV_CUS_NUMBER field that I set up as a FK?

Once I create the FK....then when I add a new customer to the Customer's table...will it automatically put the Customers Number in the INV_CUS_NUMBER field in the Invoices table?

And what do you mean when you say "Normalise"?

Thanks for the help