You can enroll in currently available courses in just a few steps from the Class Central information page (e.g. one like this).
Select the "Go to class" button. This will take you to the course page on its learning platform. If this is your first course, you will need to sign up or register with your name, email and a password. Some course platforms allow you to sign up via social media, or may ask for more contact information as well. Once you have signed up, you can click on the Enroll or Join course button to start learning.
Some courses don't have a Go to class button, but you can select Start learning. You don't need to sign up with a course provider for these courses, because they are watched directly from the Class Central site. Currently, these courses do not have a certificate.
Some instructors mention resources or links "below the video". Look for the Direct link near the top left (on your computer) or tap on the title at the top of the video (on mobile).