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There are a number of tools that can work such as small planes with curved bottom or round bottom spokeshaves. I find them fiddly to set correctly and use.
Mostly I use curved/round micro planes, files and the "sandpaper on dowel" Brion mentions.
...... round face spokeshave for inside curves .
the "round face" spokeshave is shown in this link 11P33.01 ... I just used the garrettwade link because it's the 1st one that came up as the example
shape with the spokeshave then to finish hand sanding you make an outside curve block of wood to wrap sand paper on