Decide on a supervisor, and contact the research group coordinator expressing your interest in being admitted for doctoral studies in a given topic.
Institutions have a list of PhD Programmes available in different domains. You will be informed on the institutional procedures that must be carried out in order for you to be admitted. Be ready to present a proof of academic qualifications.
Once admitted, you will be informed on the procedures to enrol in the high education institution that will be responsible for awarding you the PhD degree.