Firstly, I'd like to apologise - I do not know the correct terms for what I am asking.
Assume that the top/bottom of the highlighted portion there is actually aligned with the base.
To help explain: I need to wrap that section of the cone using a piece of paper. What shape (exactly) do I need to cut out from said paper so that it will wrap flawlessly?
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$\begingroup$If I've understood your problem correctly, I think this should help. This one's a right circular cone and its opened up paper version. Here we've taken an arbitrary curved surface of the form shown on the cone and visualized.
Since the distance from the apex to the base is constant. Assuming the cone is made of paper, cutting it open along your yellow line will make a circular sector whose radius is the former distance. Your cross-sections will correspond to bands concentric with the sector's circumference.
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