Select the layer you want to add a shadow to
Click the Layer Style icon at the bottom of the Layers panel
Choose Drop Shadow
Adjust the Opacity, Angle, Distance, Spread, and Size settings
Click OK to apply the shadow
Use the Move Tool to reposition the layer if needed
To edit the shadow later, double-click the Drop Shadow effect in the Layers panel
To create a shadow manually, duplicate the layer
Fill the duplicate with black or a dark color
Blur the duplicate layer using Gaussian Blur
Lower the opacity of the shadow layer
Move the shadow layer slightly behind and offset from the original
Use a layer mask to refine the shadow edges if needed