Keep in mind that it is only possible to rate and review your writer after your order has been completed. An order becomes completed once your warranty period ends (either in 20 days after getting the final version of your assignment or as soon as you press the button "accept the assignment now").
When this happens, you'll see a prompt to review your writer on your order page:
You need to give them a rating on a 5-point-scale and leave a comment about your experience of working with this writer. Your review will be posted on the writer's profile and other users will see it as well, so please be sincere and accurate.
Be aware that if you leave an unjustified negative review, your writer will have a chance to send a petition against it to the administration. If their petition is accepted, your review will be removed.
You can also leave a review for a writer who dropped your assignment at halfway to the deadline. This option will be available on the order page as well but only until you hire another writer.