Related Questions:
Make a pre-written email
Create emails in advance
Template emails for delivery
Answer:
In Settings, click on Templates & Files
Select Email Templates
Click the New Template button
Fill in Title, Description and Default Subject
Create the body of the email
Add any attachments, or link any uploaded files
Click Save when complete
Additional Information:
Learn more about the Merge fields you can use in your templates here
To apply merge fields in your templates follow the steps outlined here (starting at step 3.)