Drop shadow is a very simple effect and it is very easy to create this kind of shadow from virtually any object or graphic. First, create any object, picture or photograph in a layer. Then:
Create a copy of this layer.
Maker sure that new layer is under the original one.
Float the image in the new layer.
Select brush or airbrush.
Select the color — that will be the shadow's color.
Deselect image.
Then go to Filter menu, choose Blur, then Gaussian Blur.
Set required blur amount.
After that move blurred image according to your taste.
Conclusion — shadow should be in lower-right position.