Wow! Thank you for the comment! Your drawings are very well proportioned and show a likeness to your reference. Just try to go dark on the dark parts and light where the high lights are supposed to be. Overall though, your drawings show a lot of character and emotion. Well done!
. Honestly, I am not someone to learn how to shade from though. My art teacher always tells me though to practice drawing small circles with more or less pressure in like a spiraling motion. Since then, my shading has improved drasticly and it's to him that I owe a great deal of improvement in how I draw, simply look up something like 'Circular Shading Method". Good Luck!