Rinse the mustard greens and drain. Boil the water and let it cool to touch. In a large bowl, add vinegar and salt to the mustard greens. Add the hot water.

Mix it thoroughly then put the mustard greens in a jar. Squeeze as much as you can in it. Add some of the hot water into the jar until full. Close the jar tightly.

Let it sit in a warm place for 2-3 days to ferment. Once it's ready, store in the fridge. Just add salt and peppers to taste when you're ready to eat some.