Finely shred the cabbage and place onto a large bowl. Add a generous sprinkle of sea salt and massage into the cabbage for a few minutes until it starts to get juicy.
Leave to sit for 20-30minutes.
Meanwhile finely slice the orange and add that to the cabbage. Massage the mixture together.
By this stage bit should be nice and juicy, with lots of water coming out of the cabbage.
Put the mixture into a 1litre kilner jar, pressing each handful down really firmly to reduce air pockets and increase juiciness. Keep adding handfuls and pressing firmly.
Pop a weight on the top so all the cabbage and orange is submerged under the liquid.
Leave on a shelf for a few days. Do a taste test after 3-4 days and again after 7-8 days to see how your ferment is coming on.
You can eat when you like, but the longer you leave it, the more it ferments.