This works for me. I let a cucumber ripen until almost all of the green color is gone, and it is starting to soften a little. It was looking like frost outside, so I labeled this one to keep and stored it in the garage to let it finish ripening. This is a bush pickle variety. While cleaning up, I discovered in another location that I had missed an Asian cucumber, which also ripened. I cut them open. Lots of nice seeds. The flesh is watery, with a sort of pleasant, melon scent. Then I scoop the seeds and watery flesh into a jar, add some water, and let them ferment a couple of days. The fermentation removes a seed coating that dries on too hard for the seeds to take in water when planted. After these ferment 1 to 3 days, I'll wash them in a sieve and let them dry on paper towels before transferring to a paper envelope for storage. This works nicely for open pollinated or heirloom varieties, which grow true to type. I've saved mine for a number of years (plant generations) with no problems.