the best way to do it is to find a car with non rusted quarters...
i have one extremely helpful hit though that i've used a couple times with excellent results...
you can use the front fender from a honda to recreate the rear wheel wells...
lets say you have rust right behind the drivers side wheel, very common area...
you would take the passenger side fender, cut the front corner out, cut the rusted area out, and just weld the new piece in...
it usually matches perfect...
depending on your car you might need an accord/integra/civic front fender...
just see which front fender has the right lip size and curve...