When you're enrolled in a course, you can submit a review at any moment.
Just log on to your account, visit the course page, and at the top of the course menu, you'll find a link on the right marked "Rate this course!".
As you write your review, please also give it a title and a star rating.
If you're unsure how to write a review, you can think about your level of understanding before you started the course and what you know now, how long it would have taken you to gain this knowledge on the job (i.e. without the course), and what it could bring to other people taking it.