When you have reviewed your peer's article, you should leave a message on their User Talk page (unless your instructor encourages you to do it some other way). Leaving a message on a User Talk page is different than leaving one on the Talk page of their sandbox (remember that ''every'' page on Wikipedia has a Talk page), and will notify them (whereas, leaving a note on their sandbox will not).

To find a user's Talk page, go to their Wikipedia User page — something like "https://en.wikipedia.org/wiki/User:HistoryBuff34". You'll see the Talk link at the top left of the page. Click "New Section" on the Talk page and enter your message. Remember to sign with four tildes! (~~~~)