Remove the bottom part of the stalk and the upper leaves from the cabbage heads. Cut several firm white leaves and set aside. Cut the heads in half and chop into thin strips. Cut 2 or 3 cabbage heads into halves or quarters. Put part of the chopped cabbage on a cooking surface, add salt and stir slightly. Then rub the cabbage with your hands until juice appears. It is important not to overdo at this stage so that the cabbage does not turn out to be too soft. Peel carrots, coarsely grate and add to the shredded cabbage. Stir slightly. Put a slice of black bread at the bottom of a clean container for fermentation (it can be small wooden or ceramics barrels, large glass jars or enameled pans). Cover with one of the reserved cabbage leaves. Lay part of the prepared cabbage and press down thoroughly. When the container is half full, put the cut cabbage heads tightly, one-to-one. Place apples between them. Put the cabbage, shredded and mixed with salt and carrots, at the top. Press down. Continue filling the container so that a knoll is formed on top. Press down again until juice appears. Cover with the reserved cabbage leaves, then with a clean linen towel and put a tightening weight on top. Leave in a warm place. When cabbage starts to foam, make a few punctures to the very bottom with a long wooden rod. The fermentation process usually takes 10 days. After that, the cabbage should be stored in a cool place where the temperature is not above 0-2 °С.