Select the "Go to class" button on the course information page to go to edX, then select the Enroll button on the edX course information page. You must pay if you want a certificate. You need to register your name and email address and create a password with edX before you can enroll in any courses.
You can then work through the course materials and complete assessment items. Earn a certificate by gaining the required number of marks for the course.
Required pass marks can vary from course to course. Some courses have limited chances to re-do quizzes and exams, so think carefully while answering the questions.