Hello, and welcome to the forum.
Some time ago I was also interested in this car (
How a paper model was used to animate Cruella's Car )
That's a very nice item.
To answer your question: best way to do it "old school" is to start by drawing
the classical 3-views on paper.And start developing the shapes from there.
You only need to develop half of the car (one side), as the other side
would be identical but mirrored.
If you look carefully at the photos, you start to notice that only two (or three)
major components have curved shapes: the roof and the long fender that cover the wheels.
All the rest are flat surfaces (mostly) or very simple curved standard shapes.
That makes the developing a lot easier.
The hubs of the wheels are simple truncated cones.
You only need to design one of those and copy x4.
I would say that by having all flat surfaces developed and assembled from a good 3-view drawing,
curved shapes could be achieved in the classic way of trial and error.
Sure there must be some geometric solutions to them (measure the curved line of the flat side
of the fender and make sure it matches the corresponding length on the top of the fender etc.)
but sometimes for the sake of speed a "manual" fit, trim and correction is good too.
I very much would like to see a kit of this prop. Is a wonderful design.
And I believe drawing it "by hand" would be a most appropriate approach to it.
I'm sorry I didn't actually answer your question, but I hope you get the idea.
It is basically a matter of developing simple shapes. The old principles of geometry apply.
And don't be afraid. After all, Curved surfaces are nothing but a succession of small flat surfaces.
Here I found a photo showing how the card models for the film were made by hand:
