We’re going to start at the very bottom of the Layer Style box and add a Drop Shadow. Keep the Blend Mode at Multiply and the color at black (#000000). Move the Opacity slider down to 20%.
The Angle depends on where the lighting in your image is coming from. If the light is coming down vertically from the top, you can set the angle to 90 degrees. If your light is coming from the side, adjust the angle accordingly.
Keep Use Global Light checked. Fill in ‘4’ for Distance, Spread, and Size.
Make sure to set the Contour to Cone and to turn on Anti-aliased.