Creating a customer is like creating a lead. You need to click in the upper right corner of the "Customers" section, then press on the "New customer" button, a new card will appear:
There you can fill in the customer’s name, contents of all fields of the entity, you can add additional fields. To do this, click on the "Setup" tab next to the "Statistics" tab. Also, you can specify the date of the next purchase.
Just like in the lead card, you can create or add a contact or a company, fill in their fields, plan a to-do or write a note.