First you will need to make sure you have activated your student ID number. After you have been admitted to the university, you will receive a letter from the registrar's office with this information. If you have not, please call the department and make a request for your ID number. It is issued to you when you apply with the university.
Go to smu.edu/activate to activate your ID.
Next you will need to familiarize yourself with my.SMU. my.SMU is a web based application offering a variety of features for students, faculty, and staff. This is the portal for enrollment, financial statements, grade records, and much more. SMU's Office of Information Technology provides online tutorials for many my.SMU services and you can access those here.
Access the student center in my.SMU to enroll for courses. Make sure to use the class number, and not the course number for enrollment purposes.