Smack your welds before you start trying to straighten the sheet metal.
The welds themselves shrink and pull (distort) the sheet metal.
Hammer and dolly directly on the welds is where to start.
As soon as you hammer & dolly (stretch) or use a shrinker on the sheet metal, you may have a hard time recovering.
I don't claim much experience but when I filled in some holes in my fender and didn't understand the weld shrinkage I made a mess that took a long time to correct. I bought one of the shrinking discs and it worked well for me but I think I could have avoided the need for it if I had just planished the welds as I went along.
Here's a good explanation of monitoring shrink along a seam and relieving by planishing (randomly snagged online, I'm sure there are others):
https://www.bangshift.com/forum/foru...hing-mig-welds