To create a realistic shadow in Photoshop is not so difficult. For this we need just a few actions. To begin with, take a picture with a person or any other complex object to which you want to substitute shadow.
Now allocate the object and delete the background. This can be done in several ways, but in the specific case it is best to use quick mask or magic wand.
Create a duplicate layer with Ctrl + J and turn it into black and white by pressing Ctrl + Shift + U.
Next, click the right mouse button on the layer, pressing on it Ctrl + T and select from the list Flip Vertical.
You can now move the newly created layer down and apply a similar item Perspective.
Thus, we will make a realistic model of our shadow and create the desired slope and size.
Apply a blur filter to the image for this layer so that the shadow looked more believable.
I recommend to use Smart Filter – so you will be able at any time to cancel actions on the filters in the document.
If necessary, apply another transformation to the layer with Ctrl + T.