Slice cabbage thinly. This can be done with a knife, a mandolin slicer, or the slicing attachment on a food processor.
For every kilo of cabbage, mix with 20g of salt, or, for every pound of cabbage, mix with 9g of salt, about 2 tsp of finely ground salt.
Toss cabbage and salt with your hands so it is well distributed and let rest, about one hour.
Knead/massage the cabbage with your hands, to encourage more juice to come out. Add optional bay leaves (about 1 per lb) and/or caraway seeds (about 1 tsp per lb).
Pack tightly into a jar. I usually grab a handful, drop it into the jar, and then use my fist to push it down well. You can also use a pounder, or something like a thick stick.
Once all the cabbage is in, make sure there is enough liquid (from kneading) to cover the cabbage by about an inch. Weigh the cabbage down, as discussed above.
Put a lid on the jar. Be aware that you will need to occasionally let the C02 out that will accumulate.
Set it out of direct sunlight and let sit for at least 4-6 weeks. 4 weeks is the minimum required for the full cycle of probiotics to develop. Exact fermenting times will depend on ambient temperature and your tastes. The sauerkraut pictured has sat for a month, and it could use a little more time.
Put in the fridge to slow fermentation. Enjoy probiotic goodness!