As a non-degree seeking international student you are allowed to combine subjects from different schools, programs and semesters, as long as their schedules are compatible.
Approximately two months before the classes start, you will receive a list of subjects taught in the semester, including their workload and number of ECTS credits.
The course registration takes place once all schedules are available, generally one week before the semester start. You are allowed to make changes to your course selection up to two weeks after the official semester start. After that period no changes are permitted and you will stay enrolled in the courses you have signed up for by that time.