Simply put it in the skin folder you wish and go and run the game.
Depending on the car you want to use,you might need to rename the image.
The Numberplate will look like this.
Because not every modder has included a number plate on his car, it might not work on every car.
Check out my other mods as well: http://www.racedepartment.com/downloads/authors/ltcars.247335/