The short video below outlines the steps required to set up a Turnitin assignment in Canvas. The video shows how to create the assignment and add it to an assignment group called "Turnitin" in your assignment index page. It also shows how to add the assignment to a unit in your Canvas module.
The main thing to note about creating Turnitin assignments in Canvas is that you need to select External tool under the heading "Submission type" and click "Find" to search for Turnitin. You will then need to edit the Turnitin specific options in the assignment inbox after the assignment as been created.