I'm not that great at pixel art but try checking artwork from pixeljoint.com. I used to save pixel art files from there to open them up on Photoshop/Gimp. And then I would zoom in on their shading to study them as a guideline. That might be of help especially if you are completely blank on what to do. But yea, eventually you'll figure out how to shade your own way by applying what you've seen from different pieces of artwork, and by looking at how other gfx artists shade their heads on here too. The Graphics Design section of this forum is useful as well. I've seen players give tips on AA shading and etc. So I think that sub section would be the most useful to you especially if you want to learn more on Graal pixel art from real pros.