I was watching this video, which seems to be quite old but still helps. Basically it explains the character modeling process based on using extruding, subdivide and edit sops. Designing custom shapes like nose, custom circles, etcs is all manual. What I found is that the whole process is much more difficult and time consuming then what I was expecting. I created the basic shapes (still not as precise and good as I want it to be) and tried to create a chin. The result so far is not satisfying.