Instructions
Download the "Super Sonic Mod" files from the GTA Garage website. (See Resources.)
Copy the files in the "Super Sonic" and "Sonic" folders into the game installation directory's "Models" folder. By default this location is "C:\Program Files\Rockstar Games\GTA San Andreas\models." This will overwrite the files currently in that folder, so it is a good idea to make a backup of those files first.
Put the files in the "Script" folder of the mod into the "GTA San Andreas\data\script" folder in the installation directory. This is the script that will activate the special abilities for the mod.
Start "Grand Theft Auto: San Andreas" and create a new game save on the main menu. Once the game is loaded, go to the Binco clothing store located near the starting area and purchase a white T-shirt to wear. Also, go to the nearby barbershop and purchase the Afro haircut. These are both required to make the models of the modification appear properly.
Press the "Aim Weapon" and "Look Behind" control buttons at the same time to activate the "Super Sonic" mod. These buttons by default are the middle mouse button and right mouse button. Press the two buttons again to deactivate the mod.