As an assistant, you are usually engaged for six years, during which you split your time in preparing your PhD and providing services to the faculty (usually a teaching assignment). Although it will take you longer to obtain your PhD than with other types of funding, this way you are really given the opportunity to master different skills such as research, teaching, providing services. Such diversification may come in handy when orienting towards your career after your PhD.
As a PhD candidate you can apply for assistant mandate vacancies, so be sure to regularly check our vacancy page. You do not necessarily have to have the most excellent grades in order to be eligible for an assistant mandate: different PhD projects look for different profiles, depending on their emphasis. Of course you will need to be passionate about research, and in addition to enjoy your qualities being put to use in the education process of your faculty.