Tools you will need:
TXD Workshop,
Some imaging tool (MSPaint, Photoshop, Paint.NET, etc).
Getting to know the parts of the vehicle:
Back.
Roof, Boot, Hood.
Window 1 & Window 2, Front.
Side.
You are ready to mod:
This is the template I made and used as a mod on the pictures just shown ^:. Time to get out your painting tool.
After you are done:
Simply save the car as a .png file. Then fire up TXD Workshop, find and open "gta3.img" (C:\Program Files\Rockstar Games\GTA San Andreas\models\gta3.img), search and find the skin you are modding the paintjob for (example, "sultan3.txd"), and "import" your .png. There in a search bar under the list of items. Start up your GTA and enjoy!
TXD Workshop,
Some imaging tool (MSPaint, Photoshop, Paint.NET, etc).
Getting to know the parts of the vehicle:

Back.

Roof, Boot, Hood.

Window 1 & Window 2, Front.

Side.
You are ready to mod:

This is the template I made and used as a mod on the pictures just shown ^:. Time to get out your painting tool.
After you are done:
Simply save the car as a .png file. Then fire up TXD Workshop, find and open "gta3.img" (C:\Program Files\Rockstar Games\GTA San Andreas\models\gta3.img), search and find the skin you are modding the paintjob for (example, "sultan3.txd"), and "import" your .png. There in a search bar under the list of items. Start up your GTA and enjoy!