This is a video that shows how I do quick fur shading on Paint Tool Sai. It was made super quickly and it was originally only intended for one person to view and use but I decided to share it with you all. I am also a bit hesitant to call this a tutorial, but as there are some tips and explanations I guess I can call it that?
The lineart is not mine nor is it a free lineart that you can use. However, it was provided by GypsyCo on DeviantART.
You may pause whenever you need to read the text that pops up. You may also pause the video to look at the settings of my brushes and layers, if you'll find it helpful.
For shading, I make a new layer over the blank colored layer, set it to "Multiply" (for shading) and I select "Clipping Group" for the layer so that when I filled that layer with the shading color, it would not go outside the colors of the color layer. I then select the color of the character (or in most cases, the environment) and fill the entire character with that color on the new layer (sometimes I then darken the color). Afterward, I take my eraser tool (Or in most cases, my airbrush set to "erase" mode) on a lowered Opacity (usually around 50-70%) and erase away the areas where the light would be hitting.
Another thing I like to do is duplicate the color layer, make it so bright that it is white, and then I do the shading on a layer over that. This way, I can properly see what I'm doing without the character's colors and markings being a distraction