You have different options to create a customer account. It can be from the app and from your Back Office.
From your Back Office:
On your Back Office click on Configuration > Customer Account
On the upper right corner of the page click on New Customer. Add the customer information and click on SAVE
From the "customer accounts" tab of the application:
Click on in the upper right corner of your application
Fill in the fields and click on Create customer
When you want to associate your customer details to a specific order you can do as following:
Click on the round arrow on the upper right part of the ordering page and select Assign Customer
Fill in the fields and click on Create customer. Finally, select Assign to Order