oh wow, hard to explain shading, but I just practiced A LOT! Do not be afraid to really get in there and press hard to get those darker tones where the shadows are. You can soften your pressure to creat mid tones and then go back over highlighted areas with an eraser. I sometimes use a q tip to blend. But honestly, practice practice practice, and don't be afraid to fail once in a while. That;s how you learn. I am still learning new techniques every day.