INSTRUCTIONS

You can edit the paintjob or livery with a drawing software
that has transparent background options (example: 
FireAlpaca). When you're done editing, use a txd editor 
software (MagicTXD) to replace the default livery with your 
own livery and save.

MagicTXD Download:
http://www.gtagarage.com/mods/show.php?id=27862

FireAlpaca (Free drawing software) Download:
http://firealpaca.com/