Practice

Example 1: Shadow of a cube
Initial cube:

Firstly, let’s make the shadow in direct close to the cube. According to the third rule it is the darkest area. Retouch the cube itself in order it is a bit darker on its foundation, and draw the near-by area of the shadow. 
 

And now let’s add the main shadow, much blurred as it is much far from the cube.

We have a good shadow of a cube which stands on some surface under the daylight now. If we want to make a shadow from the sun or some lamp, we use third rule then. Also note that the shadow is not of a black color, as the object is of a green color, and is not much dark.

Example 2: shadow of a toy car

Initial cut image:

Recall the rule Darker and clearer if closer.
Determine which part of a car is nearest to the surface. Obviously, the wheels. Let’s build the way for the wheels – actually you should not build anything but I will describe everything by steps for your clearness – in order to determine the surface where the car stands on:

Now lets show places where wheels are nearest to the surface with green lines.

Draw the clear black shadows in the area of wheels.

Blur up and lessen the richness in those places where the wheels are not that close to the ground – recall 1 and 3 rules:

Now duplicate the shadow layer and blur it along the gauss. Because of the daylight, shadow from the wheels is situated on all sides rather than on some exact directions from the wheels. Recall the first rule. Adjust the transparency.

Further we draw the shadow from the body of a car itself. It is heterogeneous. There are places situated closer to the surface as well as remote places. Firstly, draw the homogeneous shadow, which the body of the car would cast in case it is homogeneous itself. And then simply add some shadows where needed and erase them where they are not required. Do not forget about the second rule. Still the car is bright red and may cast a subtle red shade, though not necessarily. With the help of layers styles and also color overlay, we change the black color to the slight vinous.

Note that the front part of the car as well as the left footboard is much near to the urface. Substitute them additionally, following the first rule.

As a result we have a good and realistic enough shadow of a car.

Note:
It is not a manual on how to make a realistic shadow. If your aim is to produce a realistic shadow from the real objects, design it with the help of editors as the real shadow is much more difficult quite often. And the aim of this lesson is to show shadows which look like the real ones, can be used for the designs of different sites and which do not take 5 hours to be drawn.