The key is trial and error. just keep on expermenting with new sounds, constantly tweak new parameters and just experiment with everything you can see. Also it would help if you grasped a basic understanding of sound waves (like sin, saw, square ect.)
the Operator in ableton live is an incredible softsynth. it is for the musicians who rather enjoy designing their own sound. i respect how you dont want to use premades, i feel ya on that one. but get an operator going and just work one oscilator at a time and mess with the waves of each oscilator. add some FX and just keep on expermenting. Message me if you need some more indepth help.
Good luck mang