Open your image in Photoshop
Select the layer you want to add a shadow to
Double-click the layer to open Layer Style
Check Drop Shadow
Adjust Blend Mode
Adjust Opacity
Adjust Angle
Adjust Distance
Adjust Spread
Adjust Size
Click OK
Use the Move Tool to reposition the shadow if needed
Duplicate the layer for a stronger shadow effect if needed
Use Gaussian Blur on a copied layer for a soft custom shadow
Transform the shadow layer to match the light direction
Lower the shadow layer opacity for a natural look
Mask or erase parts of the shadow if needed