I pulled the interior panel off the door and measured the width and the height of the stepped areas. After cutting the major dimensions with a shear, I set the interior panel on top of the sheet, traced the transition area between the steps with a Sharpie, and then drew an offset line that I cut to using a band saw. I don't know what that offset ended up being, since it ended up being a cut-test-repeat cycle. The door handle cutout was traced from the interior panel as well. I just made sure to cut that a little big so that I'd have some tolerance in fitting the handle trim piece, but not so much tolerance that there would be any visible gaps.
It's not my best work, but some edge trim will help hide some of the imperfections (I hope).
If I were to do it again, I'd track down a large piece of cardboard or two and make a template out of that before cutting metal. I have NO idea why I didn't do that this time. Brain no work well sometimes...