Roughly chop the cabbage and rinse.
Finely grate the cabbage in a food processor or by hand.
Place in a large pot with a lid (I use my le creuset like ones).
Mix in salt.
Then grab a potato masher and pound the living day lights out of the cabbage, keeping an even level of cabbage in the pot. This releases all the natural juices of the cabbage.
Do this for about 10 minutes or until to start to see the juices of the cabbage appearing.
Add the whey if using.
Make sure that the mixture is covered with some liquid and when you push down on the cabbage you see lots of juices appear. It will just go off it it is too dry - the pounding is really important!
Cover the pot and leave for about 3 days in a warmish place (like near the oven or boiler).At this point feel free to taste it, if you want it more sour tasting then leave it out for another day or two.
Then transfer to smaller jars if you wish or just place the whole pot in the fridge. This stops the fermentation.