Plz follow these steps:
Navigate to the “Contacts” section on the left menu bar then click the “Customers” sub-menu.
You will see a table of your existing customers. Search customer name in search box.
Navigate to “Actions”, it is present in the first column of all customer’s rows, click on it and click the “Pay” link and you will see a popup.
Enter payment amount, select payment method and payment account and click on “Save” button.
Plz watch this video: