We talk about removing side warts a lot, but I haven't seen much on the actual process. I have a good idea how I would proceed, but since so many have BTDT, I thought I'd see if you might share tricks, wisdom, and pics.

Weld in a metal disk the size of the hole. Tie off the wires.

Nothing special on my end ,just as Mark said.: Metal disc welded in.
I used the wire for the “wart” to power my front euro park lamp. So
the orange signal portion only lites in signal mode.

As you know, I like to over think... Yes, weld disc in the middle, but did you pre-curve it before to match the subtle curve of the fender, just weld in flat disc then hammer and dolly to shape after, or....? Then there are 3 additional holes to fill. My instinct is to just zap them shut using a copper backing spoon - slowly and carefully to avoid warping the fender. The goal, of course, is to accomplish without having to use filler. It looks like a simple enough task, but given the location in the middle of a thin and gently curved panel, I suspect there is a little more to it.

I have my own plans for the wires but will save that as a surprise.

Ok amateur here. I didn’t even think about the curves! I think I solved that
with filler :-( ......plus the flares on mine creep into that area as well.

Ah! I forgot about your flares. That would change things. Although my goal is always to patch without needing filler, I don't think I've completed a single patch that doesn't need at least a touch of filler. But less is more of course

Getting that curve right is a real benefit of removing the wart. Wish I had some good advice, but I'm just a spectator.