A lil tutorial thingy for how I (usually) shade/highlight! I hope it's not too confusing to understand??
I forgot to mention that on another layer, above the shadow layer, you can add in a touch of light-ish grey in the middle of the shadows to give it a more... Idk, a reflective feel I guess?? Idk how to explain it lol

Also, I used similar colors for the shading, but you really don't have to use similar colors to shade/highlight with. You could use like... A yellow base with purple shading if you wanted to! Don't be afraid to go nuts and explore with colors!

Hopefully this helps somebody at least a little bit c: If not then-- Oh well I guess! lol