I need some feedback and tips

[proper pumice]

I was making a Joker thumbnail and stuff, it looks wonky and stuff, I really dunno what's wrong with it... (I'm fixing the mouth)

[sinful frigate]

Try avoiding using a black airbrush to shade. If you still want your airbrushed look, use a dark blue instead on the layer above then use multiple. It’ll make the lighting look more realistic.

[glass widget]

I agree with this airbrush comment ^

Something you can do with minimal effort is make a mask layer, set it to multiply, fidget with the opacity. Fill the entire layer with a dark colour. Then with some form of round brush with pen pressure on with the opacity and use the transparent colour option, so it acts as an eraser. And start erasing the shadow where light will be