1. To add an existing customer to the current sale toggle to customer search field by clicking 

    Note: Customer can be added only in online mode

2. Search by typing name / email address / phone number in the search field. Select the customer from the search results.

4. When you are done with the sale, click Pay Bill and complete the Sale.

Note : If it is the new customer or else the customer details are not saved, then you can refer the following article to know How to add new customer during a sale?