If you want to paint shadow lines on a model care you should first realize that you can pair this technique up with any other technique you want to for different effects. Shadow lines occur when the out lines get thick and it provides a contrast to make something pop out. Shadow lines will focus on the light source. You want your shadow to be realistic so you want to think about where you light source will be. To paint on shadows lines, focus on where the light source is coming from and darken these areas. to make your shadow lines, bold and darken the lines by drawing them on thicker. At the bottom of a tear, you can shadow in a lot more at the tip and bottom of the tear. This will make the tear seem longer and it will give you depth. After you have painted your shadow lines you should first cut out the part that is not apart of the shadow and paint. Then you take the bottom and top of the shadow, rip this off, and paint the black shadow line.