Its fairy easy and hard at the same time. The best method is cut and plate the affected area. If the rust is on an exterior panel try and get some fresh steel from a donor car with no rust in that area.
When cutting out the rust start as small as possible and increase the hole slowly until the outer rim of the hole is fresh steel. make a plate out of sheet steel or panel and carefully weld it into place. Take as much time as you can so that you don't warp the steel.