I'm doing quite a bit of work on my truck at the moment to get the roof straight. I posted up some tools I had made to another forum and was asked how to use them, so I took a video. It may be of some use here as well.

https://youtu.be/CGVE3Do-4bA

The slapper file is made from an old bastard file. The tang is cut off and it is heated with a Mapp torch and bent in a vice to make a handle. My other one I also smoothed the handle part off with a flap disk to make it a bit nicer to hold.

You can also use some thick flat bar, as this won't mar the surface, however the cross hatch on the file makes it very easy to see the low spot rise up, when it's marked it's level.