Cut the cabbage finely - I do this with a large sharp kitchen knife, first by cutting the cabbage in half to remove the hard stem from the base, then slicing as thinly as possible.
Fill the bottle or jar about 1/4 full of cabbage.
Sprinkle about 1/2 teaspoon of fine sea salt onto the cabbage then using your fist (or a wooden bruising implement) push it down and work it until the water comes out and over the bruised cabbage - this takes some time, so be patient!
Keep repeating the above step until the jar is full (or you run out of cabbage). You should end up doing this about 4-6 times and finish with the cabbage submerged in it's own juice/water.
Weigh the cabbage down with something (such as a small saucer or glass weight, or a cabbage leaf folded over and squished into the top of the vessel so the shredded cabbage stays UNDER the juice.
Leave it to ferment on a kitchen bench or in a cupboard for about 2-3 days then start tasting - the fermentation process also eats salt so as it ferments it should actually get less salty. Fermentation should take between 3 - 30 days (usually about 2 weeks).
Keep tasting until it is to your liking and once you are happy place it in the fridge to slow the fermentation process down.