First create a new layer on a gray background, the mixing mode is screen.


Copy this layer, invert (Ctrl + I), mixing by 1 pixel down, the mixing mode is multiply.


Reduce the opacity of both layers to 20%


Colorize (the color layer in soft light mode), it can be done twice


Light up as needed (the layers in soft light mode), insert the icons, buttons, text, or whatever design requires.


The idea is taken from here: designmoo.com/resources/simple-icons/
Icons are taken from here: designmoo.com/resources/glyphs-psd/

Addition: the second step, we do edit → define pattern and further use pattern overlay in luminosity mode.