First you must leave the course an official star rating:

Once you have left it a star rating - you can write a new discussion post reviewing the course by clicking on the triple dots on the course and selecting 'Forums' on your dashboard:

Click 'New Discussion'.

Make it an official review

Finally, once you have posted a review on the discussion forums, you will get an option to make it an official review by marking it as a review:

Or if you like, and haven't left a review yet but have left a rating yet, you should be able to write a review from the course homepage also.

Please be sure to read our review writing guidelines for optimal review writing