How does a student get an override into a class that is full or for a class for which s/he does not meet the pre-requisites?
All overrides must be obtained from the instructor, and the decision to override a student into a class is solely the instructor's. To inquire about an override, the student should attend the first day of class, and ask the instructor if overrides are possible, and how they may be obtained for that particular class.
The option to override students into a course is totally the instructor's perogative.