I kept a mature bush pickle cucumber fruit to save seeds. It became very large. To save seeds, the fruit needs to ripen and start to soften, and lose its green color,
While cleaning the garden, I also found a Chinese cucumber. I didn't know it had any fruits. The seed was from an old packet, and only one germinated. These were in an out of the way location in the garden. I will save that seed too.
Both look the same when sliced open. Despite being over-ripe, the insides had a fresh, pleasant, melon scent.
The seeds get scooped out, along with some juice and loose flesh. I place the mixture into a jar with some water, and allow it to ferment a few days. The fermentation removes the seed casing. Then they'll be rinsed and allowed to dry.