This is for Gp3 only. If you want to edit the official addon (Gp3 200 season) ask again.
All you really need is a decent paint programm, like Paintshop Pro or Photoshop and a Gp3 Editor (Gp3Edit or Gp3Master).
You extract the texture files with the editor, save them as bmp and edit them with the paint program. If you browse the download section of garndprixgames.org you might find blank templates to start from.
After you have down the painting, simply reimport the bmp files back to the game with the editor.
Did I forget something? O yes, you need some skill and patience, too. Good luck.
REHAB IS FOR QUITTERS