1. Trace out the object you want to shadow as close as possible (you might want to zoom in as much as possible)
2. Create a new layer with about 75% opacity(less depending on what it is)
3. Fill the area in with black
4. Use perspective transform to morph the shadow to fit the surface it's being casted on
5. Use liqufy to bent and distort parts of the shadow to allow it to conform to any odd terrain