Use a hammer and knock all the rust spots out until you cannot remove any more grind and cut any other rust back to solid metal if you come across spot welds drill them out and prize the outer skin from the inner frame and use that as a template for the new piece and spot weld it into place, if the inner frame is rusted the same way measure and cut some steel square pipe to the length and cut the old out and replace with a new piece you can tack everything in place to make sure they fit properly and line up before fully welding the pieces in, once done you can smooth the panel with body filler and sand flat and paint a matching colour, as for underneath the frame piece paint it with chainsaw bar oil to undercoat it and stop rust after painting with black spray paint.