1. In Settings, click on the Create Canned Message button. 

2. Give it a name you will easily recognize. (other agents will not see the name of the message)

3. Add the content or body of the message.

That's it. Now everywhere you can send a message, you will be able to use a canned message if you choose. If you choose to use a canned message it will override the system generated default message if there is one.