In many ways the most important person in the whole process of obtaining a doctorate is your supervisor (promotor). He or she supervises the research and gives advice. The supervisor has to declare that he is willing to accept you as a candidate (promovendus) before any further steps can be taken.
 
Every full professor has the right to act as a PhD supervisor. By the time you plan to earn a doctorate in the Netherlands, you may already know - from publications in academic journals, conferences and similar gatherings - which professor you would like to have as your supervisor.