Navigate to the booking for which you want to send an email to. Click the eyeball icon to view the booking.
In the top right of the booking, click "Comms."
In the row "Custom" click "Email" at the very end. A box below will appear and you can type in your email to the customer here. It will have the same layout as the emails sent by Booking Phoenix.
Click "Send" once you have completed your email.