When I traditional I just scribble very light lines in the spot I want to shade and smudge it with my finger
If I need it lighter just erase and smudge finger again
In digital I just add a new layer and set opacity to 20% and just use black airbrush and eraser in combination, both set at 90% density
That's just for the cartoon ones, the lineless ones I've been doing recently it's too hard to explain
Oh and the only pencils I use are standard no.2 I don't use artist pencils lel