Go to "My work" -> Rate & review".
On your pending Wips to rate, you will see two buttons: "Rate" and "Skip rating".
Press "Rate". You will be able to assign a rating from 1 to 5 stars and a field leave a review.
Press "Save".
NOTE: Using the grey dropdown menu on the left, go to "My Work" -> "Supplier Dashboard". You will be able to access the "Reviews" tab to see the reviews you received from your customers.