In this exercise, I'll be adding a perspective shadow to the two objects above. You can use just about anything for this tutorial, as it is very versatile.

As you can see they sit on a 1 point perspective plane. The shadow will be falling on the floor plane and be cast from a single light source.

1) It's pretty important to figure out where your light source is going to be as it helps determine how "long" or "short" the shadows will be cast from the objects.

For a 1 point perspective grid such as this, it must be exactly on the vertical plane above the vanishing point like shown above. The higher up it is, the shorter the shadows are and it's the opposite for a lower light source.

If you want a different shadow angle, just move the single convergent point to a new location. Make sure to move that light source with it.

2) Next, if your objects are oddly shaped, draw a square that exactly surrounds the objects. After you've done that, select them then click on the Guides layer to turn them into Guides.

Grid hidden for clarity.

3) Turn on Snap to Point. Next draw a line from the center of the light source down to and beyond the innermost top corner of each of the Guide boxes.

When I say innermost, I mean use the top corner that is closest to the light source like shown above. Select these lines, and click on the Guide layer. Again these lines will be turned into Guides to be used later.... you'll see.

For now, hide those Guides.

4) Next create a new layer and name it Shadow. Put this layer below any others like shown above at left.

Second, select and Clone the objects casting the shadow and put it on this new layer. Give this clone a Basic fill containing your shadow color and set your strokes to None or use that shadow color again.

5) While this clone is still selected, choose the Perspective tool. Click-Hold on the clone and tap the Down Arrow key to project it to the floor plane of the grid like shown above.

Don't worry about the size yet... you'll be enlarging it in a moment.

6) Now we are going to flip this clone. Click-Hold on the clone again, and tap the Space Bar to flip it vertically like shown above.

7) Now to enlarge the shadow you must use some Perspective Grid modifier keys while holding the mouse down on the object.

First Click-Hold on the shadow and press the 2 key to enlarge it both x and y. To speed up the enlarging process, you can hold the key down and watch it enlarge. This could take a while to happen depending on how large you need the shadow to be.

You can move the shape around while you are doing this to get a better idea of how much more you need to go. Also, letting up on the mouse once in a while will help with placing the shadow like shown above.

8) Now turn those Guides back on. You'll be using the vertical line guides for determining how far out that shadow will fall on the floor plane.

Since you used the top innermost corner of the square as a guide point, you'll be using the bottom innermost point of the projected shadow bounding area.

9) Using a similar method as in step 7, instead we'll be using the 7 key to enlarge the shadow in the y direction only.

With the Perspective tool, Click-Hold the shadow and press the 7 key to start enlarging the shadow. When you extend the bottom innermost point of the object to hit the vertical guide (highlighted above), you're finished.

10) Now repeat steps 4-9 on any other objects you want shadows for. Like I did for the second person shown above.

When you are all finished, you can select the shadows and go in the View menu and choose Perspective Grid -> Release with Perspective.

After that, you can give the shadows special touches like gradient fills or even use Lens fills if they cast over other things.

Other modifications to this include raising the objects casting the shadows to appear as if they're floating.