I'm in awe of anyone who has a knack for this. Let's just say I find it challenging, difficult, tricky, and a few other words...
I'm plugging away about an hour at a time every few days.
After 3 sessions on one side, I was feeling like I was making progress and had smoothed out a lot of bumps and ripples:
Then I looked down the length and found smooth is not the same as straight. I had created about a 1/4" dip. The crown that runs the length of the part was too flat where it dipped, so I figured I needed to stretch it to try and restore the radius. I did this slowly, using the slapper on dolly, not the hammer that gets me in big trouble.
I also used a short length of pipe on the back side, a dead-blow hammer, and some manual pursuation to try and restore the radius. This all worked well enough and the panel is much straighter:
The curve is now close to matching the other side. I'm not saying the other side is a good model, but it's what I've got:
The other side is where the part was folded but I'm learning a lot as I fumble through this... filler will be my friend.
Mike