Course organizers are free to select students based on all the criteria they have used in the past (including type of study, courses that students should have attended before the seminar, motivation letters, etc.). Please specify all your prerequisites explicitly in TUMonline and do organize an initial meeting as you always did. You can still collect all the information from students that you need outside the matching system and then prioritize those students, who satisfy all requirements.
If a student submits a preference for your course, he confirms that he satisfies the requirements and knows the information from your initial meeting. If it turns out after the matching that this is not the case, your are free to remove him or her from the course.
Once you have decided on the students that you want to have in your course, just assign them the top ranks. You can also assign multiple students to rank one or two if you are indifferent. If you do not want students in your course (e.g., for lack of prerequisites), you will be able to mark these students such that they will not be assigned. In this case you will have to give a short explanation why the student is unacceptable. Otherwise, just assign these students a rank. The system will not do random assignments of students any more who did not express a preference for your course. This was done in the first pilot to make sure every student is assigned a course seat, but it was considered a bad idea.