Related Questions:
Use an email template
Populate an email with a pre-written one
Procedure:
From the compose email pop-up:
In the 'To' Field: Ensure a Contact with an email has been selected
Optional: add any additional contacts by using the BCC and/or CC fields.
Fill in Document, Event & Invoice information if needed
Click on the Select a Template dropdown
Select the template you wish to populate
Review & edit your email body as needed
Click Send.