After your course as well as trial finishes, you will receive a report from your tutor.
You can check your report on the [My Couses (Student)] > [My Reports] page

in the [My OnTutor > Student] menu.
You can just simply click the [Leave a Review] button on the report, and rate and leave a review.

Or you can always go to the [Course Status] > [Completed] page in the [My OnTutor > Student] menu and click the [Leave a Review] button.