First thing you do, and I MEAN this if you want to learn this stuff, toss that "shrinking hammer" in the trash can. Or, take you grinder and grind the head smooth as I did with the one I mistakenly bought years ago.
They are a JOKE and do nothing but destroy metal.
There are many youtube videos on shrinking but honestly, I had some serious disagreements with every single one of them. This one is as close as I could get to just your basic shrinking.
There are some other guys here with a lot of Tin skills that will likely pop in on this one. But for a basic quick shrink the video shows it ok. DO NOT put the back side of the hammer on the panel, why on earth the guy is doing this is beyond me. Your very basic shrink is heating a tiny spot then cool it with air or a wet rag. This is your basic, you can get a lot done with it. Quickly get the torch right to the metal, heat it to red or near red and pull away and cool, it's that simple.
Where you would do this is another story, on the middle of a hood, you are going to be in deep trouble.
The next step would be where you see the guy hitting right around the edge of the heated area "Pushing" metal into the most heated area.