Practice on shading, the middle looks so messed up because of how I got lazy and made the lines of darkened colors MUCH larger. The yellow X is my light source, for anybody wondering.

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How can I improve my shading? I seriously want to get it fixed up before I make another sprite.

That, and a couple suggestions for how to get started on line-art for a much larger sprite.

It's gradient, and uses WAY too much shade.

How should I shade it, instead?

Depends on what it's for. Usually 3-4 shades per color is enough.

or using the base figure that Damian made.

How should I shade it, instead?

I'll use your Gambit avatar as an example. See how his coat only has 3-4 colors on it? It has the outline color, the base color, and then the 1 or 2 colors that are shading it to show you shadows. That's pretty much how you'll want to do your shading when you do your sprites. Sometimes shades may require more than 2 colors but it isn't often.