In textures, we're drawing implicitly vs explicitly. In cast shadows, there is never just a line on it's own, there's always a thickness to the line.

You want to look for the shapes that those shadows make, and because we're using fineliners there are no shades of greys or mid tones we can use to show the transition, we have to choose how much of that dark grey we color. As you get to the next step and draw your transition, you would color more of the grey closer to the left (darker side) then transition into only coloring the deepest bits of shadow on the left/lightest side.

I've had a quick paint over your paper example for the shapes I saw, your crumpled paper casts erratic, tapered shapes https://imgur.com/gE1KZJp

Do your best not to draw what you see, but the shadows you can observe.