How do you make curved folds in origami?
Apart from just carefully folding along the curve with your fingers, following techniques get used:


Embossing the line with a pointed object or (better) a used-up ballpoint pen. Folding along such an embossed line results in a relatively smooth fold even if the curve is complex.
For some models (mostly the organic-looking ones), wet folding can allow very smooth lines to be folded. This is sometimes combined with scoring the paper using your fingernails. Daniel Chang uses such a technique for folding his realistic human faces.
Robert J. Lang famously uses a laser tool to score the crease pattern into the paper. I was able to fold from such paper in one of his workshops and the scored line looks a row of tiny perforations which weaken the paper and guide your hand when you fold along the line.
Some folders (e.g. Ekaterina Lukasheva) use digital plotter-like tools for embossing the paper.


Machine-embossed lines (the latter two examples) are of course much more precise than those made by hand, but even a line manually scored using a ball point pen can be surprisingly precise.