This is a pretty good video with a lot of good information for patch and sheet metal work with a mig.
That IS a good video.
I have done sheet metal patches and quarter panel skin jobs and his method of cutting at the angle seemed wrong at first but made sense once he showed how he did it. You need a gap between the panels for the weld to go, I thought his 45 degree angle would leave too narrow of a gap but it worked out much better than I expected. I use those sheet metal pieces he pulled from his toolbox. I may try it his way and see if I'll even have a use for those in the future.