If you are going to shrink using hand tools, then :
* There are many ways to shrink. Many methods. Using hand tools and machines. I have identified 14 distinct ways to shrink. You have to adjust your mind to the concept that when you hammer on sheet metal with a steel hammer onto a steel dolly - or anvil - that you will expand or increase the surface area by thinning the metal. You also have to grasp the fact that you can reduce the area of sheet metal by thickening it. Metal flows. Get used to this idea and your abilities will grow.
* Heating up metal and throwing cold water on it may shrink it a tiny bit, but it makes distortion and hard steel. Why not just heat it and gently hammer the swell down? This works, the other doesn't.
* DO not over-heat the steel. Heat to blue only and shrink that much. You can do it again, if needed, but over-shrinking can be more of a problem than under-shrinking.
* DO NOT quench red hot steel, or blue-hot steel with cold water. It hardens the steel and only makes your job worse. Let it cool on its own, then when it only sizzles the damp cloth you can quench it down to feel the panel.
* Don't waste your time with "shrinking hammers" or "shrinking dollies" They do not work. You will waste both your money and your time. All of the pros know this, and the people who sell this junk are only interested in your wallet contents. They do not care that you waste your time and beat up your work. You are nothing but an uneducated cash cow to them.
* When using a "grab-and-shove" machine (Lancaster, YME, ERCO, Marchant, Eckold, or whatever) it is best to start at the edge and make a pass, then go deeper with your subsequent passes. Keep the jaw teeth clean.