Go to JOGUStINe, click "Vorlesungen" and select the semester you are looking for on the left-hand side. The courses are listed by the institutes which offer them. Under "Studienfächer" you can see which institute or course of studies belongs to which department.
Complete the registration form with all courses you wish to take and print out enough copies so that you have one for each institute and one for yourself. Take one copy to the "Studienberater" (academic adviser) of your major and have him/her check and sign it, then take the signed and stamped copy to the "Studienbüro" of your major. If you have chosen courses from more than one course of studies/institute, repeat this process (first Studienberater, then Studienbüro) for each institute separately.
Please note: you will only be signed up for the courses once the "Studienberater" have signed your registration forms and you have turned them in to the "Studienbüros" until October 23rd 2015 latest.
Your course registration is binding and cannot be changed any more once the registration process is completed.