Put the sliced cabbage into a non-reactive bowl, add salt and seeds and bang it around with a potato masher or meat tenderizer, until it starts to soften a bit.
Pack it into a quart jar, using the end of a wooden spoon to really force it down.
Top it off with just enough water to cover.
Let it mellow for a month or more, occasionally releasing any gasses that collect in the jar. Just be warned, it will be stinky. If any bloom starts to develop on top, scrape it off.