Willow-Heart here it is!
1. Clip a new layer on top of your color/lineart folder
2. Block out the shadowy half of your drawing with a dark color (I used red of this one)
3. Lower the opacity to your standards
4. Go to the filter button
5. Go to blur and choose lens blur
6. Do the same with lighting but with a light color. I used yellow
7. On a new layer (keep clipping all new layers) sketch out on your preferred opacity where the light would hit. Then blur it twice
8. Do the same with the shadows
9. If you're feeling fancy, add extra lighting to some places and change it to "add"
Hope this helps!