To do this put the jar of seeds and juice in a reasonably warm place for 3 days, stirring the mixture twice a day. It should develop a coating of mould and start to smell but after 3 days add plenty of water to the jar and stir well. The good seeds should sink to the bottom of the jar.
Gently pour off the top layer of mould and any seeds that float. Then empty the good seeds into a sieve and wash them thoroughly under running water. Shake off as much water as possible, and tip the sieve out onto glass plate (the seeds tend to stick to anything else). Dry somewhere warm but not too hot, and out of direct sunlight. Once they are completely dry, rub them off the plate and store in a cool dry place, where they should keep well for at least 3 years.