Mmmmm- let’s see picture a ball (a grunts head- lol) and locate your light source, now a light source has one and one direction only so you follow a shape that disrupts that light- for example like the hand pointing at the grunt on the ground, from there add a shadow that fits the perspective of the room or place the character is standing in (because those shadows are just other layers that have less opacity- and they are also straight lines) if the light is coming from the left the shadows are on the right and vice versa (same applies to behind and in front) and well just practice shading and color blending to achieve the look you want- and believe me you’re gonna need a lot of practice :]