Convert surface patch to a sheetmetal and unfold
The surface, you have visible in your model, is composed of compound surface angles and therefore cannot be flatten using the sheet metal tools present in Fusion 360.
John Hackney, Retired
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As John has advised this can not be done in Fusion 360. I ran it through Meshmixer which has inbuilt flatten or unwrap as they call it but dropping the derived shape back into your file...Not at all sure that result is trustworthy. One thing I can see is the flat pattern will not vary all that much from the silhouette of your Surface Body. (Attached for your interest).
