Other than that it is just copying the files in "textures_high" over to the same folder in the game folders. Copy the "textures high" folder from "pox - EKS Ekstrom/Heikkinen over to the game directory folder where you find the "textures high" in the car you want to replace.
Looks like a high-quality adaptation! Personally I would change the Audi/Quattro logos with something a bit more Volkswagen/generic. But that's just me, the skin looks great anyway!
http://www.racedepartment.com/downloads/dirt-rally-mod-manager.8062/
Other than that it is just copying the files in "textures_high" over to the same folder in the game folders. Copy the "textures high" folder from "pox - EKS Ekstrom/Heikkinen over to the game directory folder where you find the "textures high" in the car you want to replace.