Answer:
I don't exactly know if this is the correct way but it works for me.
Explanation:
I gently scribble some lines, dark or light scribbles depending on the darkness of the shade you want, then I use my finger and push down whilst spreading around the scribbles making it look like shading. If there are still scribble marks afterward, you can erase it and gently do it again until it matches the shade you'd like. I don't know if this helps but that's the technique I use
