this isnt really a tutorial on how to make a skin, this a tutorial on how to make a template for a skin build with the extra factors. It requires knowledge of how to manipulate graphics. but to answer the question. there a several ways to eliminate that grey background through layer blending. but actually what you need to do in this instance is make the grey into white through contrast and brightness changes whilst retaining the clarity of data and assigning the layer as a multiply blend. As its white and due to the nature of white with this blend option, the white will appear invisible thus not affecting any other layers. and obviously these layers must be above your hand created skin job. (If you left it grey and used the blend it will darken your texture).