This tuning has long been a favorite of old-time banjoists. To get there from the “open G” tuning used for most bluegrass banjo, just drop the 4th string down two half-steps from a D to a C, and raise the 2nd string up one half-step from a B to a C.
This was also the first tuning that I used for solo banjo composition. I don’t use it often anymore, because it lacks the low bass notes of the tunings I prefer now, but it’s still a lot of fun to work with.
The following videos use double-c tuning: