Step 1:
Open your image in Photoshop (File >> Open).

Step 2:
Create new layer (Layer >> New >> Layer)

Set foreground color to black.

And select a soft brush. And set diameter little smaller than shadow size you want to add.

Click once just below the shape to which you want to add shadow.

Step 3:
Go to: Filter >> Blur >> Motion Blur

And apply following motion blur settings.

Pres ctrl + F 2-3 times to re-apply blur motion effect. Bottom shadow effect is added to your image. See the image after adding bottom shadow effect using motion blur.

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