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.
How to save cucumber seeds so I don't have to buy them from year to year.
How to harvest, prepare, and store melon and cucumber seeds for next year's garden...
How to Grow Cucumbers in your Garden- Tips for growing cucumbers, including how to plant cucumber seeds and how to transplant, care for cucumber seedlings and more cucumber gardening tips.
Saving cucumber seeds More
Growing Cucumbers in Container Gardens -- Good start-to-finish guide (best to use 5 gal containers)
Seed saving is rewarding and cost saving to the home gardener, but some seeds take a bit more TLC to save than others. Cucumber seed collection, for instance, requires a little bit of knowledge to properly save. Learn more in this article.
Organic Lemon Cucumber Heirloom Vegetable Seeds by nimblenitecap, via Etsy.
Socrates (F1) Cucumber Seed - Johnny's Selected Seeds
'SOCRATES' cucumber seeds from Johnny's Selected Seeds company / 7" long / suitable for growing indoors with temps 50-82F / resistance to powdery mildew