option one (if its a image type file) -
first download spark iv for your version of gta iv or gta iv eflc , open spark iv.
find vehicles.img and open it and find where your vehicle slot is eg. polpatriot.
edit the wtd file and import your skin there under the name of the orignal skin eg.paintjob
and save and close, then save.
option 2 (if its a wtd file) -
first download spark iv for your version of gta iv or gta iv eflc , open spark iv.
find vehicles.img and open it and find where your vehicle slot is eg. polpatriot.
insert your skin into the wtd file.
click save.