When you receive a feedback request you will get a notification in your Nav Bar as highlighted below.
If you click the bell you will see a drop down list of your notifications as shown below. To see the feedback request click the notification.
You'll then be taken to the feedback request. You will see the request itself as highlighted below, who requested the feedback and who the feedback is regarding.
To respond to the request type into the text box. When you are happy with the feedback you have given click 'Submit Feedback.'
If you'd like to learn about declining a feedback request click here.