Once you have found a supervisor for your PhD project and you both have signed a supervision agreement, you should apply for admission to the PhD procedure.
The application for admission to the PhD procedure is usually submitted in the PhD office at the beginning the PhD project with the necessary documents (Application for Admission Link). Admission requires that the person interested in the doctorate fulfils the requirements for PhD studies according to § 4 of the doctoral regulations. (Admission Requirements Link).
In addition to the PhD committee, which checks the formal requirements, there are other services for PhD students.