You'll find many different opinions. Your options range from a very basic 'wash and brush on waxoyl', through 'wash, wire brush, apply rust converter, paint, apply something like dinitrol with a spray-gun' to 'get it done professionally'.I've concentrated on the chassis, the inner wings and the rear axle.
Taking into account the value of my vehicle, which is pretty rough around the edges, I've always opted for the first option and try to do it once a year. Some will say that is the worst of all worlds because if you don't clean off the existing rust, all you are doing is sealing it in. However, I see the waxoyl as providing a barrier between the chassis and mud/road salt.
My chassis was in good nick when I bought the car 50,000 miles ago and it's still in good condition now. The rear axle has now had some welding on the turrets. The inner sills have now had to have some welding too, so the basic-waxoyl approach didn't stop them rotting (but they might have been a lot worse if I hadn't done anything).
In retrospect I might have spent more time over it and cleaned and painted before waxoyling. It wouldn't have taken much longer really.