It's embarrassingly easy... I use "Fido" brand jars (normally the 5L size) as their gaskets allow pressure to escape without letting mould/bacteria/yeast in. They're also cheap in comparison to kraut crocks.
Finely slice some cabbage (red, green, or whatever type you want) and place in a large mixing bowl. Give each 1 pound of cabbage about 10g of natural sea salt (don't use the fine stuff - anti-caking agents stuff it up). Massage it very harshly - you want to start breaking up the cells and fibres. Get rough - it's therapeutic. Put it in the Fido jar and push it down hard (use a potato masher, a rolling pin, a baseball bat, your fist, or whatever). Repeat until the jar is NO MORE THAN 3/4 full (as it will expand over the next few days. You have been warned). Tip any brine from your mixing bowl into the jar. Close the lid, leave it at room temperature and out of direct sunlight for a month. Then open, enjoy, and keep it refrigerated from that point on.
One special note - it is possible for pressure to force some liquid out of the jar, so I sit the jar in a bowl. It's easier to clean the bowl than half the kitchen bench and floor...
Use the same approach for grated carrot, turnip (which gives a delightful horseradish aroma and flavour), or whatever you like. The other option (which I use for chillies, carrot, etc) is to put finely sliced veg into a room-temperature brine of 3.5% salt. Again, one month, room temp, out of direct sunlight. Adding some garlic and/or ginger does wonders.
I really should make a video the next time I do this.