Pretty much make patterns, templates, cut the patches-often slightly oversize,
Lay them in place, hold down with screws or clecos, cut through both layers (the patch and the repair area)
At the same time but only partially, tack weld while pushing down/holding the patch flush with a screwdriver or similar.
Slowly cut and tack until you're all the way around each patch, stitch weld, then grind using anything from
An angle grinder to sandpaper rolls/cartridges, to mini (13” belt) sander.
Welding and grinding takes some time, fitting the patches has to be right and you
must cut into good metal.

Fitzees fabrication on youtube is a good resource for doing this without much in the way of special tools.

I use mig because I have appropriate equipment for sheetmetal and experience with it, but for the highest possible quality I feel that
oxy-acetylene with jewler's torch and/or tig (possibly a combination)
is most likely the final frontier for larger jobs because the weld is a much softer composition, less stiff, and easier to work with...however the
tedious nature of the job will be amplified with other methods.