'39 Chevy 2dr Sedan
I made a little grid on file folder and taped it to the trunk edge. I then took a photo that matched the reference photo and overlaid them. I can plot the trunk on the grid and transfer this to some styrene. This is also a good time to locate the taillights.
I trimmed the grid along the plotted line and transferred it to the styrene. I made two identical pieces and glued them along the trunk edge.
Plotting the reference trunk to the grid.
I found this Chevy on the web. Another overlay shows me where the window goes. Some frosted (low tack) tape with some pencil lines over the rear window area.. Again I add  a few random dots to help me count the lines. The bottom edge of the window fell on one of the dots. I'll fit the window to match.
Turned out that the top edge was already right. Scratched the window outline into the body and cut that baby out. Add glue.
Sort of a clapboard system. Evergreen to the rescue. 1x4 mm strips.
There's not much I can say about putty. There's air-dry, two-part, automotive and paintstore stuff. I use Plastic Wood and Elmers Wood Filler. Spread it on and sand it off. Karate Kid style.
Next is to sand it off,
"Putty on, Putty off"
That's it for cutting. The rest is putty and sanding.