Do you mean cars with a new 3D model or just new textures (skins) for cars that are in the game?
If you mean just textures, you need Driver Texture Editor (DTE), start it up, open the two files that correspond to the city in which you replace the skin (each city has its own textures for the cars), for example "NY.lvl" and "NewYork.d3d" or something like that for New York. Find the textures you have to replace, go to Import and find the textures to replace (one at a time of course), then save. This is for driveable cars, replacing textures for traffic cars is just slightly more complicated.
If you mean new 3D model, the only one I've installed is the 70s Firebird (
http://www.drivermadness.net/forum/view ... =72&t=2074) a long time ago and I think I remember it had pretty clear instructions on how to do it. So I guess that one is good for starters. You need DTE for the textures as well.
Im not very good at tutorials, sorry if I wasnt clear

so do ask again if you have any doubts, mostly about textures. Good luck.