Wash heads under running water, shake excess water off.
With a large knife such as a French knife, quarter the cabbage heads.
Cut out and discard the cores.
Slice the quartered cabbage into slices about 2 mm (.07 inches) thick (the thickness of a US / Cdn quarter or a 10 p piece.)
Put cabbage either into a very large bowl or directly into the container you are going to use to ferment it in - whatever you prefer to be working in.
Sprinkle the salt over and mix in.
Some sources say to let it stand a bit so the salt will start drawing at least a bit of liquid out. Some suggest 10 minutes; the All New Ball says cover it and let stand for an hour.
Begin pounding / crushing the cabbage, and keep on crushing it until a good amount of liquid has been released from the leaves.
Ideally, you are going to be aiming for enough released liquid to cover the cabbage once in the fermenting container, but you may rarely get it at this point -- don't worry.
Add to fermenting container, if it's not already.
If you are doubling, tripling or quadrupling the recipe, start the next batch of cabbage now, then add to container.
Repeat until you have prepped all the cabbage you planned to.
When you are finished crushing, if the cabbage is not already in the fermentation container, move it all there.
Be sure to leave a gap of 10 to 12 cm (4 to 5 inches) between the cabbage in the container, and the top rim of the container as headspace.
If you have more cabbage than that, you'll need another container.
If the juice you were able to produce was not sufficient to cover the cabbage, then make a brine (see directions below) and use that to top up with. (To be clear: you are not aiming to fill the container to the top rim with liquid; you are after enough liquid to completely submerge all the cabbage by 2 to 5 cm / 1 to 2 inches.) FIRST, though, see Linda Ziedrich suggestion in Brine section: she suggests waiting 24 hours to see if it will produce enough juice on its own before adding the extra brine.
Cover cabbage with some sort of weight (see Weights Options below) to keep it safely below the surface of the liquid.
Cover container with a large towel.
Let ferment for 3 to 4 weeks or until cabbage is translucent.
During fermentation, you may or may not need to check the cabbage frequently. It depends on the weights you use (see Weights Options below).