single player, you say?
Save the modified skin as a .bmp file in C:\Program Files\Rockstar Games\Grand Theft Auto\skins\
Go to options » player skin setup.
Select your new skin, click ok (or double-click the skin).
Return to the game.
Can be changed before starting the game or while playing (pause). And btw, if the skin doesn't reload when restarting the game, enter the skin selection menu when restarting the game and exit it...