To send an email to a customer, you will need to firstly go to the orders tab as shown below. Once you have found the order, for example in 'Awaiting dispatch' as shown below, then you can select the edit pencil icon in the far right column.
When you have clicked on the edit icon, you will be taken to the order edit page. Here you can send an email to the customer by clicking the 'Send Email' button. The email box will appear where you can edit the email content and add a email subject. Your custom emails will appear in the drop down menu as highlighted. You can then choose the appropriate message for the customer and press then press the send button. Once the email has been sent it will be saved to the order for future reference.