I struggled with this too. I get about 75% of it and I think there is 25% that is rather too ad-hoc or expedient. My tips would be.

Forget about local colour. Just try to think in terms of shadows.

Then because something is either black or white you need to think about what threshold you are using before it swaps from one to the other. When doing a gradient, this threshold will change from one end to the other.

Do not expect literal copies of the reference, that is not what is going on here.

I struggle with some of the choices in the demos. IMO The hairs, some of the lines on very smooth surfaces are not really shadows, they are shapes or colour variation. However adding them helps interpretation of the object. This is what I mean by expediency which should be used sparingly.

With all the above said. The process of doing this is really useful. It makes you look very closely at what is going on and forces you to make interpretations and judgement calls almost to the point of abstraction. It's a frustrating and difficult exercise but the end justifies the means for me. It really helped me develop my eye.

So, when you are going through hell, keep going.