Duplicate the layer by right clicking with ctrl and right click then click duplicate
Make sure the image in front and color is set to black
Now fill the new layer by pressing Ctrl+Shift+Delete if your on windows Mac:Cmd+Shift+Del (The shift key will make it only fill where there are pixels aka preserve transparancy)
Filter>Blur>Gaussian blur, amount of 3-5
Now to distort the shape instructions below
Press Ctrl/Cmd+T, this will bring up the free transform tool. Right click on the selected object Mac:Cmd click. Select "Distort" from the pop-up menu.
You will see a bounding box with 8 little squares, click and drag the top left and top right squares this makes a square around your image (called handles) Press Enter/Return to apply the transformation.
Duplicate the shadow layer. Drag to the icon again.
Apply the gaussian blur again this allows you to blur the picture, this time add a larger setting 5-10
you now want to make the shadow softer
Go back to the top shadow layer and add a layer mask this is shown to the right
click the gradient tool drag it from left to right to blend the two shadow layers.
Finally Set the shadow to multiply mode and the opacity of the top shadow layer to about 50% and the bottom shadow to about 30% this allows you to experiment with how dark or light you want the shadow