Cope, cut and dowel, probably. Otherwise, the hard way - trot out the
carving tools, scribe, cut, trim.
That said, we're all pretty used to seeing such a connection uncoped, why
not go that way?
I am attaching chair arms to spindle back supports with mortise and tenon
joints. Is there an easy way to make the end of the square arm part concave
to match the curve of the back spindle since there is a tenon in the middle