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In the top right hand corner click on the Settings Gear and click on Settings in the drop down menu.
Under the Settings heading, click on the Labs tab located in the center of the page
Under Available Labs, find Canned Responses (Second one from the top) and click Enable
Click on Save Changes
See Below For Demo
Once Canned responses are enabled click on the Compose
Type the message you’d like to have saved for your canned response in the message window.
Click on the down arrow in the bottom right hand corner of the message window next to the Trash can and hover your mouse over Canned responses.
Click on New Canned Response
Enter in a name for the canned response (Example: Vacation) and click on OK
Anytime you’d like to use your canned response, click on the down arrow next to the trash can, hover over Canned responses, and select the response you’ve created and it will automatically fill in your response in the message window.
See Below For Demo