see this?

i just grabbed it off some screenshot of some mod
you can take a screenshot of your sidebar your self
now select one of the blank squares like that and copy it
now i paste it onto what i want

\/
and change the blending mode to screen
and get this
use the eraser tool on a large soft-edged brush
and erase away the bits you dont want
i used an ar2 cameo and did this pretty fast, so the gloss isnt properly aligned and there is a dark outline(ar2 icon style)
but i hope you get the idea
you dont have to do this everytime you want to make an icon, in PS you can drag layers from one file to another, and then readjust it, or else you can make your own template psd file and open it everytime you want to make an icon