In order to leave fan mail for another hubber, first you need to be following them (which I see you already know how to do).
After you are following a person, click on their profile page. Scroll down past their published hubs and favorite topics and look for the "Submit Fan Mail" field.
Enter your message in the text box, click on one of the icons below the box (ex: You're cool), and click on "Post Fan Mail".
Once the person you've sent the fan mail to reads it and approves it, it will appear on that person's profile page.