To leave a visit on another member's profile, just visit their profile—from a member list, yearbook, Classmates search results page, etc.
If your profile visit setting is Normal, your name will be left by default for each profile you visit. (You can remove your visit on a case-by-case basis.) If you switch your setting to Quiet, your name will not be left automatically. You'll have the option to leave your name by clicking Tell [your friend] you visited near the top of their profile.
You can adjust your setting at any time under your Account. For desktop users, click My Classmates® in the top portion of any page. (For mobile users, click , you may have to expand to make your selection).
Select My Account, Privacy and then select your setting (Normal or Quiet) next to Visit profiles anonymously.
After you've left your name, it will appear in your friend's Classmates® Guestbook, along with the date you visited.