In order to save seeds from cucumbers, you must let them thoroughly ripen on the vine. They will enlarge and turn yellow. They should stay on the vines until the vines are dead. Bring the cucumbers into the house and let them ripen further on a dry shelf in the pantry (or someplace out of direct sunlight). When the cucumbers begin to turn soft, scoop out the seed mass and put it into a large jar of water.  Let the seeds ferment for five days, thenseparate the scum from the good seeds that have sunken to the bottom. Rinse the seeds in a colander, then dry hem on screens for at least three weeks, or until the seed snaps when bent in half. Store the seed in airtight containers, label and date clearly. Store the containers in a cool, dark place free of humidity. Seed processed properly will remain good for at least eight to 10 years.