Note: I am Still learning about shading myself. It's super tricky to learn shading. This is just what I know so far.

1. Do not shade/Highlight with Black and White. Shade is made of color.
This was the Video That Helped Me Learn THat Shades are Made of Color at about 9minutes in: https://www.youtube.com/watch?v=9cnd-ikZG8o

2. Identify What Color the Light Source Is. (Blue for Morning/Midday, Oranges and Yellows for Afternoon,
Is there a Yellow Lightning bug as the source of light?, etc etc.) The Light source is what color you will be Highlighting in,
but in a Pale color. (Pale blue, Pale yellow, Pale orange, etc) The Shade Tends to reflect the color of the Background.
(Try to use Colors that you've already been using in your Artwork. That way it will help bring things together)
If no Background, then Default DEEP Blue or DARK BROWN depending on character.
(Still Working on this Myself. Not sure what colors to use where sometimes.)
Atroquine Does Artwork Often that Reflects using Colors for Shades and Highlights.
I Recommend stopping by one of their Art Streams.

3. Identify What Direction is the Light Source Coming From.
Where is the Light Coming from determines Where things are lit up and where things are Shaded.
The Direction of Light will have the same force in Shadow on the exact opposite side.
I Think of Light and Shadow as two entities fighting for space, (but amorphous, unlike my cruddy green sketches in the image).


A Trick for Cel Shading. (Or shading in general)
1. Make sure the entire image area you want to shade is Selected
2. Paint Bucket the Entire Area The Shade Color.
3. Use the Eraser Tool and erase the Areas where Light would be.

Cel Shading this way helps your eyes pick out where light is happening rather than
trying to remember where shades might go. This was the Video that Helped me
to learn that: https://www.youtube.com/watch?v=nQcbnTlYhF4&feature=youtu.be

Well... I hope this mess could help someone. X___X
This is Alll I know about shading besides "Light and Shade make the Lines"
but I don't know how to explain that statement I just said to people.