Trying to weld, I’m terrible!

Like already mentioned, mig likes clean metal, N rust or oil or anything and get a good ground. And remember IF your old sheetmetal side is thinner than the new metal side, thnk about favoring the thicker side somewhat. You will find using .023 gas wire INSIDE away from any breeze will make life easier but I use flux .030 out of necessity it works but welds are not as clean either.