During the first week of the semester, you use the Banner Self-Service online system to add/drop courses. Simply informing your professor isn't enough.
To use Banner Self-Service, you will need the five-digit CRN (Course Registration Number) for each course you wish to add or drop.
You won't be able to add a course on Banner Self-Service if the course requires permission or if an exception is involved (such as allowing a junior into a course restricted to seniors). You'll need to go through the Registrar's Office, even if you have permission from the professor.