Let's start with the car's installation:
There are 2 ways: I.Simple
Download this program : SAMI
After installing it, launch it and set the Game Directory and set the location of the mod (must be .ZIP or .RAR)
Press next and again next and now select what car should the program replace(Car to Car is correct, but Car with plane would crash the game)
II.Advanced: In the advanced section you can edit everything but harder.
You have to download this program.
Open IMG Tool 2.0 and click FILE>OPEN, and open "gta3.img" located in "/GTA san andreas/models"
Click COMMAND>REPLACE, and select the files from the mod and the rebuild archive from "COMMAND>Rebuild".
You are done...Enjoy!