For creating a new customer, click on customers option in the contacts section in the left menu bar.
Then click on the add button on the top right side of the customers page.
Enter credentials of the customer and customer group. You can add the opening balance of the supplier. Opening balance of a customer means the amount that the customer already paid before purchasing to us and it can be on a daily or monthly basis. And scroll down and click on the save button that will create a new supplier.