Teachers can create a custom canned response for exercises from the "Help" tab for a given student exercise. At the bottom of the Help tab is a link to "+ add custom canned response".
Once they have created the custom canned response, that response will be available from the Help tab or Fast Grade for any exercise of that type. For example, if a teacher creates a custom canned response on The Two Towers, it will be available anytime the teacher is grading The Two Towers.
Custom canned responses are tied to individual exercises, so that custom canned response the teacher made for The Two Towers won't be available for Breakout, etc.
To find and use custom canned responses in Fast Grade, they can be found on the left-hand side of the page, below the message box. To apply a custom canned response, simply click on the response you'd like to add and it will appear in the message box. You can also use the keyboard shortcut "shift + 1" to speed up the process.