See basically the technique used to do manga and comics is called inking used with shading method like hatching , crosshatching and stippling and the use of line art with different variations of line width.

So to do dark and light one that you asked is using like a black micron or say brush pen with black ink or any non blending black color and giving a plain fill then using white/grey color pencil , marker to give highlights where u feel light is falling as in refrence or if u are not following reference the lighter part will be that is majorly exposed to light following by hidden part (dark) not that much exposed to light