Understand.
Here was my process. No kidding either.
I drink a few on Fri or Sat night and play some tunes. No one is allowed in the garage. The old metal is cut out. Then the old metal is laid onto the new patch. I clamp them together. This gives me a perfect trace and pattern. I cut the panel and use the old piece as a straight edge, perse. Clamps, clamps, clamps to the body. I have even used self tapping screws at the bottom lip to keep it held into place. You MUST stitch weld. You must use more PSI of shielding gas. You MUST use .023 size wire, You need to take about an hour to weld across a good stretch of metal. Stitch weld 1 second or less at a time in the middle. Then stitch at the end, then the other end. Then in the middle of the 1st weld and the end weld, then do the same on the opposite side. Take a break, drink beer. You continue to stitch in the centers of your areas until there is no cut area left. THe problem you will encounter is wanting to rush as the stitches get closer.....and you think.....I just need to weld a couple here, there and thats it.....then it warps. Drink more beer......weld, more beer, weld. Same goes when grinding down the welds to be smooth as well.....beer, sand, beer, sand..... p.s. Use a flap wheel....easier than grinding wheels.