When submitting a Google Document/Slide Presentation select the submit button:
and then select the tab at the bottom that says "Google Drive":
Then you will be able to access your drive (you may be asked to give permission to access your drive or to log into your drive at this point). You can type in the name of your document into the search box or scroll through the files that are brought up. Select the assignment and click on submit...then click on submit again to turn the assignment in to Canvas.
Can't change a rubric once you've started using it.