Go to the first class. Introduce yourself to the instructor. Enrolled students who don't show up might lose their seats, so you could be in luck.
Look at other sections. Your ability to get into a lecture course will typically depend on seats opening in your specific discussion/lab section, so look for open seats in other sections too. Consider attending more than one to increase your odds (but remember, you can only waitlist one section).
Have a backup class just in case. Make sure to go to it so you don't lose your seat or have to play catch-up later.
Whenever you're told you can get in (either via email or directly from an instructor), they will then issue an electronic override. You must then go into Wolverine Access and officially register for the class. These override permissions do expire, so check your email often!