After you finish your stay in a host, you will receive a notification, in your inbox, that you are able to review your host.
Just click on it, and write your review.
You have a 15 days period to write a review to your host. And you will only see his review after you do yours, or after the 15 days period. So don’t leave it for later!
Reviews are extremely important, for both sides. If you get a positive review from your host, you are more likely to be pre-approved by hosts in future. If you leave a review for a host, they know what they can improve upon & other travellers can accurately learn what the experience will be like.