If you understand block sanding, you get the idea. First thing is to be sure the part fits the contour of the panel, then start working out the dents. You don't need a form but you do need to work deliberately but carefully.
Start by roughing in the shape with tools made from steel bars and ground to the approximate shape, kind of like a dull chisel - we used to call these "corking tools". Do not try to hammer out the shape directly with the hammer, you need control over where your blows land so it's much more accurate if you position your corking tool and hit it with the hammer. Back up the area with a steel dolly or anvil. Avoid stretching the metal.
When you get the shape close use a fine file to "block sand" the area. Then start picking up the low spots. Be careful not to file through because if that happens it's over... you will never be able to weld a hole in metal that thin and even if you did the welded area becomes impossible to work. Backing with a steel dolly or anvil will help avoid picking the metal too proud of the desired surface. Once you have the shape back, finish with progressively finer grits of black sanding paper (the wet-or-dry stuff) starting with about 180 grit and working up to 400 being sure you remove all of the scratches from the previous grade. Then polish.
I would seriously suggest finding a scrap trim part and practicing on that. Metal finishing is metal finishing regardless of the metal but SS trim is paper thin compared to a fender, but once you get the feel of it, it's not really all that intimidating.
