First, you must decide where the shadow will appear.
We'll use a ball as our first example.
Here, the light source is in the top left corner.
So we know the shadow will be on the bottom right side of the ball.
The shape of the shadow will also depend on the shape of the ground surface.
Here, we’ll use a flat surface, so the shadow will be flat.
The ball is also partially in shadow. First, we’ll draw a small reflection here to mark the part of the ball that is directly in the light.
Now, we’re going to draw a line of separation between the light and shadow.
This line marks the darkest part of the ball.
The shadow is not as dark on the right side.
The bottom right side should be a bit brighter because of the reflection from the ground’s surface.
The difference between the line and the two parts on the right must be visible but subtle.
It gets progressively lighter as you approach the reflection.
Now we’re going to finish the contrasts on the ball.
The shadow on the ground will be darkest where the ball touches the surface.
We want to make the shadow circular, like the ball.
You should draw the shadow going from dark to light.
If the ball is positioned in mid-air, the shadow will be drawn differently.
The light source remains the same, so the shadow on the ball will not change, but it will have a different shape on the ground.
The outer edges of the shadow will be the darkest part.
For another example, picture a towel hanging on a wall, slightly away from the wall’s surface.
The light source is still in the same place.
In this case, the shadow is also darker around the edges. And it has the same shape as the towel.
Don’t forget that the shadow always takes the same shape as the object it comes from, and that only the ground surface can change this shape.
Also consider the source of the light. The closer the light source, the clearer the shadow. If the light source is further away, the shadow will be more vague.
The size of the shadow will also depend on the position of the light source.
To practice, watch your own shadow as you walk down the street. The shadow cast by your body grows and shrinks according to the position of the light source.
Study this, and practice using it to draw shadows.