Add 2% salt to the weight of the cabbage and massage it with your hands or pound with a sauerkraut pounder until the cabbage releases its juice.
Place into a 3L fermentation vessel like a mason jar.
Press the cabbage down firmly so that it's covered with its own juices.
Put a piece of plastic wrap or another cabbage leaf on top to ensure no air gets to the cabbage.
Place a weight on top. This can be a small bag filled with water, ceramic fermentation weights, glass fermentation weights or anything else you can find.
Close the container or Maison jar with a lid, a lid fitted with an airlock or just a plastic glove. Ferment at room temperature for about 7 days or until you are happy with the level of sourness. When you close it with a lid when using a Mason jar make sure to let out some gas after about 4 days and repeat every 4 days after that if you are fermenting the cabbage for longer.
Notes
Wash and clean all equipment and your hands well before starting, to minimise the risk of bad bacteria.
If you want the cabbage a touch sweeter only add the sugar when it's done fermenting. If you add the sugar before the bacteria will just consume the sugar and speed up fermentation so the sweetness will be lost.
Keep the cabbage submerged in brine all the time.
If any strange mould grows on it then throw it out. It's safer to start over.
When it's ready it will have a nice tangy sourness with no off flavours. If it tastes or smells bad throw it out. Something has gone wrong in the fermentation process and bad bacteria crept in.