Perspective Shadow Effect

Open the photo you want to add a shadow to. I am using the photo below I got from a Photoshop blog site:

Make a selection of the woman with one of the lasso tools . I used the magnetic lasso tool because it clings to the edges of an object but you can used any one you are comfortable with. The selection does not have to be highly accurate because it will be used as the basis for a shadow that will have a soft edge. Here is what your photo should look like now:

 

If your selection is a little larger than the photo you might want to feather the selection. If so you would go to Select > Feather and apply a Feather radius of how many pixels you want the selection to adhere to. I am not going to feather my selection because the area selected is pretty close to the actual picture.

If your selection is smaller than the photo itself you can expand the selection by clicking Select
> Modify > Expand:

Create a new layer by clicking the new layer icon on the layers palette:

Name it Shadow by double clicking on the layer and then typing in the name.

Select the paintbucket tool and click on the foreground color in the toolbar to open the color picker dialog box:

Select a gray color from the dialog box. Click OK. Fill in the selection (Alt + Backspace).

Deselect the selection (Ctrl + D). Go to Imatge > Tranform > Distort to bring up the Distort bounding box.

Click and drag the top-left handle of the bounding box at a 45 degree angle toward the bottom:

Do the same with the top-right corner handle:

Click the green check mark when you are satisfied with the results. Drag the shadow layer under the picture of the woman.

Make sure the shadow feet line up with the woman's feet. This is my final result: