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Let the cucumber remain on the vine until yellow. Cut it and scrape out the seeds. Put the seeds into a small container of water for a few days.
Any seeds that float up to the top are no good and should be discarded. The ones that remain on the bottom are good. Pour out the water and save the good seeds. Rinse them and put on a paper plate to dry. When dry, label them with the date and put in a resealable plastic bag. Store in a cool place until you will need them. I put mine in the refrigerator.