Image for Editing
Copy the bottle image, create a new canvas on Photoshop from Clipboard and paste the image.
Use the Quick Selection Tool to select the bottle. Copy it (CTRL+C) and paste it (CTRL+V) so you have another copy.
Select the new layer with just the bottle on and no background on the right hand side of the page.
Layer --> Layer Style --> Drop Shadow... (Click okay on the dialog box that pops up)
Layer --> Layer Style --> Create Layer (Click okay on the warning dialog box that pops up)
You now have the shadow in a separate layer so you can select the layer and CTRL+T to Transform it (move, rotate, resize etc.).
You can also skew it using Edit --> Transform --> Skew
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