Wow you're right, your linework, forms and anatomy really are soo good! For shading it really helps to keep the environment in mind (even if you won't actually draw a background). For my example I drew the light source behind him, slightly from the upper right side. So I colored in the light where I imagine the sun will hit him. Then for the main shadow, I colored everything in what the sun doesn't hit, in a neutral color which I took from the environment (here the sky). When a light source hits a surface it creates "bounce light", which literally bounces off the surface. Here, it is the grassy ground. So, some light gets reflected from the ground onto him. This, I colored in a slighty green color, where I imagined it would be reflected onto him. Then for the darkest shadows: those are the places where neither the main light source nor the bounce light will reach. I really hope this helps. Shading is really complicated and it takes a lot of practise. It always helps to have some reference pictures!
Wow! Thank you so much for this! Really appreciate the effort and the advice. Definitely feel more confident to give it a go. It always kind of makes sense in theory and then falls apart in practice, but I'll try it out! And this is great reference!
I'm glad I could help you :)

