In order to comprehend the shading effects in a drawing, it’s very useful working on the perception of the depth. Adding shadows on a flat drawing helps our perception in seeing the three-dimensional effects of the depth. It’s a sort of “magic”: we can understand the different layers just if the shadows are in the right spots.
I created this drawing with students of grade 7th. We started drawing the first layer with a pencil. We drew some kind of perforated membrane, like a ribbon with rounded corners. We have sketched few layers of these membrane and we have shaded them with colored pencils. The trick is to create a shadow every time that a ribbon goes under another one. The ribbon above creates a cast shadow on the ribbon below, so we can see the depth of the layers.
We used a different color for every layer: after we have drawn 3-4 layers we have colored the background with a black marker, to emphasize the depth.