Open your basic icon. This should be square image, usually 256 x 256 pixels and will need to be a .png file with transparent background
Resize your icon. This should be dead easy - select your image using Ctrl+A, then go to Edit>Transform>Scale. You can either use Shift and the drag handle or just type 90% into the height and width boxes at the top of your screen. Then drag your icon to the top of the canvas.
Duplicate the icon by right clicking on the layer in the Layer Pallette and selecting Duplicate Layer.
With this new layer still selected, go to Edit>Transform>Flip Vertical to turn the copied icon upside down.
Still with this layer selected, hold down Shift and click and drag the upside down icon to below the original, leaving just a tiny gap.
Right click on the layer in the Layers Pallette and go to Blending Options. Once you get there, change the Opacity of the layer to 50% (under where it says General blending).
In the Layers Pallette again, click the button second from the left on the bottom of the Pallete to add a Layer Mask. A layer mask is an invisible black-and-white mask that sets a layer’s transparency - black is fully transparent, white is fully opaque.
Select the Gradient tool (Ctrl + G) and make sure you select the Black to White gradient. Next, click at the bottom of the reflection and drag the gradient upwards to the top of the reflection.