Carrameow,
As most have said..practice.. practive.. practice. Mig welding is really the simplist form of welding once you get a technique down. You need to get use to working with a welding mask on while keeping everything lined up.
I have done a few restorations...The last one was the Austin Mini which I used nothing more than a Craftman Mig Welder with .030 Flux Wire (no gas) to do spot and bead welds.
Maiting surfaces have to be rust free...ground or filed shiny clean!
Old appliances like stoves, refrigerators, computer casings are a great source of 18 guage steel....and its actually better quality than whats used on the cars. I mention this because it looks like, in your pictures you will need to fabricate some new maiting surfaces / flanges for the new panel to weld to. Be creative!...don't be intimidated.... Steel is great to work with!
Grab a book from the Library on welding...you will probably learn more technique from a little reading than from a few tips on this board. I always give myself a refresher course and practice welding before I tackle a new project.
__________________
FRED
Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K