1. Login to My Library Account
2. Using the search box on that page, search for the specific item you wish to rate.
3. To rate a specify item, click on the star that would correspond to the star rating that you would like to give the item. For example, if you think that book deserves 3 out of 5 stars, click on the third start from the left.
4. To view all items that you have rated, while in your library account and click on the My Ratings button located on the right side of your screen. You can modify or delete your ratings from that page, too.

You can also add a rating during a regular search, but just clicking on the stars where you see them displayed. You will be prompted to login at that time.