[mongocanfly]

From experience...it can be that hard.....if you've never welded thin metal don't practice on your good parts...get some old junk fenders and do a lot of practicing...if you have access to the backside you can use copper for a backer...it'll help keep it from blowing thru..
There's a couple of ways to join the old to new metal...I've used panel clamps but you'll have a narrow gap to weld up...this is where the copper is nice to have..some people only do butt welds....this'll require very fine fitment to pull this off....don't overlap your patches and then weldem up...you'll also need to know how to properly grind your welds and do hammer and dolley work