You can rate a booking by going to your booking page. If you are also an instructor, make sure that you click the customer toggle to see all your bookings as a customer. Then click the booking which you want to rate. Under the feedback section, click the + button to rate the booking.
Note that you can only rate a booking, which has already taken place. You have 14 days after the end of a booking to rate it. After this rating period has passed, you can no longer rate it.
The instructor will receive a notification when you create a rating.
If you want to learn more about editing a rating, then please read this article.