separate the leaves of the cavolo nero from the stems and wash them. If you have small, tender leaves just wash them as they are. If you have the classical long leaves, then you need to check if the stems are too hard and stringy. In that case, discard all stringy parts, but keep them aside for a mixed soup. Remember to always wash your vegetables carefully, changing the water 2, 3 or more times, until the rinsing water comes out clean (at least apparently). You may want to blanch the leaves in hot water for a few minutes, but only if the are big and hard. Keep the water. Finally, cut the large leaves to even pieces, according to your preference.