1) Make sure the extension is installed/activated.
2) Adjust various settings such as choosing your Payment Gateway, your currency symbols, and etc. at WP-Client-->Settings-->Payment Gateways, and WP-Client-->Estimates/Invoices-->Settings.
4) When you create a new Invoice, you are given multiple options. You can assign to specific Clients or Circles. Additionally, you can choose to drag-and-drop your previously created Items into the Invoice or add new Items. You can also set a due date, and type a unique message for the client.
5) After creating and filling out the Invoice to your satisfaction, just click "Save and Send Invoice". The invoice will be assigned to the client, and a notification with an attached PDF will be emailed to the client(s). The email notification will also include (by default) a login link for the client. In order for the client to pay the invoice, they will need to login to their portal (a direct payment link is not provided in the email notification).