first you open up your tube (element) you want to use I am using an element from Leasha's (Un-Leash-ed Scraps) Into the Spring Kit
1.open a new image (whatever size you want to work with) copy and paste your picture onto the new image and resize as needed
2.go to effects....3d effects drop shadow and use the following settings (or play around with them to your liking) make sure that DROP SHADOW ON NEW LAYER IS TICKED
3.make sure your layer is on the shadow layer and click on the deform tool
4. use your mouse and drag the shadow layer down with the deform tool and use the SHIFT KEY to pull the shadow LEFT OR RIGHT
and now you have a realistic shadow under your element (you can also merge down your image layer to your shadow layer)
you can play around with the drop shadow settings to your liking (blur/opacity levels)