Step 1: Locate the assignment that requires the peer review and click on it. This will bring you to the assignment details page where you should have already submitted your own work to be reviewed. If you have not submitted your own assignment yet, you will need to do that before you are able to complete the peer review step of the assignment.

Step 2: Find the Assigned Peer Reviews header on the assignment details page.
This link should be on the top right of the screen, but depending on your screen size, the Assigned Peer Reviews section could be lower on the page.

The Assigned Peer Reviews section will show you which submissions you will need to leave feedback on. Depending on how your instructor set up the assignment, you could have one peer review to complete, or multiple. The peer reviews could also be listed as anonymous or the student's name will be displayed. depending on your instructor's preference. 

Step 3: Once you find the Assign Peer Reviews section, click on one of the peer review links to leave your feedback.

Step 4: Clicking on the peer review link will open up the submission details for your peer's submission. To view the submission and to leave your feedback, click on the View Feedback link.
This could be confusing since you are leaving feedback, not viewing feedback, but to leave your feedback you must click on the View Feedback link.

Step 5: Clicking the View Feedback link will open up the submission, and you can now leave your comments on the submission. You can also leave overall comments by using the Add a Comment box on the right hand side of the window.

Below is a video tutorial on how to leave peer review feedback: