Phlearn makes it too easy. His video has just vertical coke boxes.
But the OP's glasses are photographed in a certain perspective with an angle.
That means you have to put the glasses and each of the earpieces on a separate layer to create a shadow.
Let's start with some screenshots.
1. Make a duplicate of the original image on a new layer.
2. Rotate this layer and align the bottom of there glass horizontally.
3. Duplicate this layer and flip it vertically.
4. This layer you have to divide in 3 separate layers. One for the glasses and one for left earpiece and one for the right earpiece.
You now have to rotate and distort both earpieces like in the screenshot. (on the glasses layer paint of the earpieces on the layer mask).
5. Join the 3 layers back to one layer. Desaturate the color completely, and soften it with gaussian blur.
Reduce the opacity and paint on the layer mask to achieve the desired light shadow.
6.Last step: Flatten the layers and rotate the image back to it's original angle.
This is the final image.