Choose Tools > Style (from the Tools menu at the top of your screen).
Press the S key on your keyboard.
In the Tool Options pane, click Add Style and choose Inner Shadow. You can add as many of the same styles as you like.
Customize the Inner Shadow layer style:
Blur: Move the Blur slider to adjust how much to soften the edges of the shadow. You can also enter a value from 0 px to 100 px.
Distance: Move the Distance slider to adjust how far the shadow is from the object. Hold down the Option ⌥ key to extend the range of the Distance slider. You can also enter a value from 0 px to 400 px.
Angle: Move the Angle wheel to change the angle of the shadow. You can also enter a value between 0º and 359.9º.
Color: Click the color well to choose a color or click to pick a color from the image.
Opacity: Move the slider to adjust the transparency of the shadow. You can also enter a value between 0% and 100%.
Blend Mode: Click the Blend Mode pop-up menu to choose a different blend mode.
If you have applied multiple Inner Shadow layer styles to a single layer and you’d like to rearrange them, simply drag the style up or down the list of styles.
To reset all styles applied to an image, click Reset at the bottom of the Tool Options pane.