You can write a review for a skill that you've had a booking for by going to the skill's page. Below the other reviews, click the + button to write your own.
Note that you can only write a review for a skill, for which you've had at least one booking. This booking must have already taken place.
The instructor will receive a notification when you create a review.
If you want to learn more about editing a review, then please read this article.