01. Click the Customers> Order menu.
02. Access the request you want to process.
03. To send e-mail to your client informing the status of your request, thank you for the purchase and any other information you wish to pass, type the e-mail in the "Comments" field, or if you prefer, choose an e-mail template Which email template? Check out: Tools> Email template)
Choose the Order Status.
05. In order for the customer to be notified and receive this email, click on the square in notify client.
06. After step 3.4 and 5, click refresh. The typed email will be sent to your client.

Video: https://www.youtube.com/watch?v=N2ubyXZtp_0&feature=youtu.be