You'll notice that one side of the shape is light and the other is darkened. All you need to know or keep in mind is just that. What the light doesn't hit, it is darkened. In this example, all you have to do is picture the light source and the object which is being hit by the light -- which part will the light hit? How far will the light go?
You also have to ask yourself where is the light source coming from? If the light is far above, the shorter the shadow is (try checking out your shadow at noon - 12:00PM) whereas the lower the light, the longer the shadow will become. According to the light source, make your shadow fit accordingly.
Good thing to remember also: what is the shape of the object I'm giving a shadow? Each of the shapes in the picture each have their own unique cast. The triangle has a pointy shadow, the circle has a circular shadow, the cylinder has a rectangular shadow, and the cube has a "L"-like shadow. At a different angle, though, the cube will cast a different shadow shape. For instance, if the light was head-on to one of the flat sides, it will cast a square to rectangular shadow depending on the light source's height.
With that in mind, you also need to remember: what is the shape of the object the shadow is falling on top of? The current example only has a flat surface on which the shadows fall but in most cases, shadows of - say a character - will fall on rocks or on water, which will look different compared to each other.
Drawing the Shadow
The shadow takes on the shape of the item it comes from. If you look to the example picture to the left, you will see various shapes and their shadows being cast. Notice that to make the shadow, all you have to do is create a triangular shape from the top of the object to the ground and back to to the base of the object.
The cube is a bit more complicated as there are two and in some cases, three triangles you have to draw when at an angle.
Drawing the shadow on the ground, I drew in dotted lines to indicate the shape of the shadow - which is basically the same shape as the object itself. Again, you'll notice that the cube does its own unique cast as indicated at the bottom example. It forms an "L" type shadow.
Shading on an object usually starts midway into the object as shown on the cylinder, cone, triangle, and cube. The circle is also shaded midway but considering that it is round, the shape of the shading also becomes rounded! The result is something quite like an eclipse. Note: Shadows depicted in example image are intentionally drawn entirely dark for tutorial purposes.
Source and Shadow
The shape of the shadow is also affected by the light source. When the light source is from anything but the sun, like a light bulb, the shadow widens the further it is from the object. The sun, meanwhile, casts a "striaight" shadow in that it remains true to the objects shape.
More Than One Light Source
A shadow is made for each light source present in a scene. If you are inside a room, for example, and there are two lights on, you will cast a shadow from each light source. This is shown on the right-most example.
Notice both of the bulbs are at the same distance and height from the object. This fact causes the shadows from both light bulbs to be the same. When the light sources are from different distances and heights, the light source that is closer to the object gives off the darker shadow.
Looking at the example again, note that the area where the two shadows meet is darker than the one shadow itself. Dark + Dark = Darker. Keep this in mind when drawing groups of people who's shadows happen to overlap and intersect each other.
Those basic shapes I have mentioned in the previous pages makes up the human anatomy. The circle, rectangle, triangle, cylinder, and square. The arms are basically cylinders, the head is an oval on top of a cylinder, etc. From there, the shadow is based on those shapes. Of course it's a bit more complicated since the face isn't all one shape but a combination.