Haha, everybody beat me to Fitzee. I’ll still throw in my recommendations for must watch vids of his though.
https://youtu.be/6Fvjq9Xie8Y
https://youtu.be/_u31t13QO6A
That guy is a genius and his tips will honestly make you a better welder/fabricator with practice.
I don’t have any specific MIG tips as I only TIG stuff now, but if possible I always butt weld like Turbo69 mentioned. I usually try to cut near some sort of bend or “structure” if the job allows. It’ll limit your warpage. Not on a bend, just near it so you have the advantage from it. Skip all over with your tack welds so there are no surprises. Also be mindful of your grinding afterwards. You can hand file some heavy welds down or use a fresh 40 grit briefly instead of grinding that same spot for 5min with with worn out 220 and nearly making it turn red. Sometimes it also helps to practice on an even thinner gauge to hone your skills. I did so much 19ga recently that it makes the 16ga I’m doing now feel like plate.
Biggest tip I can think of would be the fit-up. If you can make your new piece fit in your cut out area with a near friction fit without seeing daylight, your job will be way easier. 3M makes Cubitron .030/.035 cut-off wheels that are worth every penny.