9 Bringing in a shadow layer

After I decide on the light source’s location I create a shadow layer between the line and flat colour layers. I set the layer to Multiply and reduce its Opacity. Avoid using black/grey for the shadows; using a colour adds more life. I try out several colours before deciding to use magenta. I clean it up with the Eraser tool, which is set as the Technical pen or Soft airbrush for softer gradations.