A stud nail welder is basically a tool that welds a small stud to the metal surface to perform dent repair. This stud provides something for the t-handle or slide hammer to grab on to. Force can then be applied to remove the damage. When using this method, you must have the repair area ground to metal as it will not work on painted surfaces. The tip must make direct contact with the metal to produce the weld. When using this method or any method, remember the rules mentioned earlier. First in, last out and roll the metal, don't just force it. Starting at the outsides (indirect damage) weld studs to the metal. Then pull the damage as you use a body hammer to tap lightly down on the highs. Sometimes the highs may be difficult to find, but there are usually high spots. If not, I still recommend tapping lightly while pulling to relax the molecules in the metal, resulting in the metal being straightened with less force. Once the damage begins to pull out, start pulling towards the center of the damage (direct damage) using the same technique. I would also advise you to only use the slide hammer as a last resort. It is very easy to overstretch the metal using the slide hammer. I prefer using the t-handle and body hammer. Once you have completed the pull, you will need to remove the stud nails. This can be done by cutting the studs with a pair of dykes and grinding the surface smooth.
Keep in mind that the metal of newer cars is very thin. Therefore, you do not want to remove too much metal, making it thin and weak by over grinding. Grinding also produces heat, which is something that you want to avoid too. Keep the grinder moving around and do not hold it in one spot too long. I have actually had students shave door handles without warping the panel during the welding process. Then they get carried away with the grinder and warp the panel severely. To prevent these problems you may consider using a DA sander to remove the paint. However, a grinder will still need to be used to grind the studs smooth.