Cut black radishes into any shape you like (julienne, slice across, cube, whatever). I julienned mine here; in Russia folks like to slice them across into thin circles. I don't bother peeling because I think the black specs look pretty in this ferment.
In a mixing bowl, toss black radishes, salt, cider vinegar and sauerkraut juice, if using, with your hands, mixing and squeezing well until liquid starts to come out.
Transfer the salted radishes into a mason or fido jar, cover with a cabbage leaf and place a weight on top.
If liquid is not completely covering the radishes, just give it a bit of time - within a few hours they will release enough juice to raise above your weight (radishes are 85% water).
Keep it at room temperature 4-5 days, then transfer the jar to the fridge.