In order to create auto ranking systems, you should have a bot in your group, which can do this ranking by itself instead that your staff members are doing manual ranking. You can try to use the Ranking Services. one them of them called RanKGun, which provides you free ranking services for your group, you can’t find any other ranking services which can provide you ranking features for free.
Link: Rankgun - Ranking Services,Quiz Centers, Automatic Ranking, Chat Commands, Member Counter & more