Gradients offer excellent control to build up shadows. Light determines the shadow's characteristics. Hard light produces hard shadows. Soft light produces soft shadows. Shadow definition and intensity decrease with distance.
A drop shadow is included in the layer's effects options. This shadow option works well with simple shapes. The shadow distance, angle, hardness, opacity and color parameters can be adjusted to create a base drop shadow.
Drop Shadows Reshape
The drop shadow distance, angle, hardness, opacity and color parameters can be adjusted to create a base drop shadow. Then, the base drop shadow is separated from the layer effects and reshaped manually.
Select - Copy - Distort - Mask
Extract the subject from the background. Duplicate extraction layer and fill with black. Transform a duplicate layer with perspective controls to place the shadow. Add a negative mask to the shadow. Apply a gradient to the mask to reveal the shadow.
Smart Object Shadow
A shadow can be applied to similar images by creating a Smart Object shadow. The SO shadow is transferred to files alike. The Smart Object is updated to match the particular file.
Shadows can be painted by hand using a table and pressure sensitive pen. The Brush tool shape dynamics allow control over the quality of the painting strokes.