The simplest way to add a Customer is from the Customers page itself.
Using the top left menu simply navigate to the Customers page.
On the customer page click on the + Add a Customer button to start adding a customer.
You'll then be prompted to enter the Customer's Name.
The Customer will then be created and you'll be able to edit in any relevant data you'd like to the customer.
1. Editing from here will allow you to add Primary Contact information, a primary location, and whatever Custom fields you have setup for the customer.
2. If a Customer has more than one location you'll want to add them to this tab here, either manually or through an import via Excel spreadsheet.