HOW DO I - Convert a front facing image into a top-down image for maps?

[Urist]

As has been said, information about the top doesn't exist so you will need to make it up.
With pen and paper, I would put the front image below where I want the top-down image, then transfer vertical edges up. Then draw horizontal lines connecting the vertical edges based on how far you think they are depth wise. Lines that you wouldn't be able to see are not drawn or drawn as dashes. Curves, of course, complicate things.
Here is a quick example I did in inkscape.


On a computer I would likely try to convert to a line drawing, then use inkscape to save it as a .dxf, then import it into a 3d CAD program and make into a 3d model and use that model to generate a top-down image, but there is usually a little bit of a learning curve when working in 3d.

Then all that needs doing is textures.