First download the skin. Than you need open gimp. Than you can open a your skin as 64/32 - normal skin.Than you need open a template as layers - you can download shading template here :) file:///C:/Users/user/Downloads/toms/Template.png 
now you need check the right way   ------->       to Layers-Bushes down that you can see the mode. tike this      mode:Normal click that button, than you need push a Grain extract mode. and here you are. Than you need click colors and Brightness and contrast. When you have that you can control your shading :) theres a video you can see https://www.youtube.com/watch?v=59vKQqO0VfU Bye Comment and click diamond!