If you grow your own cucumbers, the easiest way to get seed may seem to be to harvest them yourself. This is not always the best method because many modern varieties of cucumber are hybrid and their seed may be sterile. Find out the variety of cucumber growing in your garden before you invest time and energy into harvesting its seed. If the garden variety is a seeding kind, you are in luck.
Wait until the cucumber has turned a golden yellow color to harvest them (often after the first frost has killed off the vines). Then store them in a cool, dry place for 2 weeks to allow them to continue to ripen.
Use a sharp knife to slice the cucumber lengthwise. Then use a spoon to scoop out the seed mass in the center of the cucumber and place the seed mass into a plastic container.
Leave the container out, at room temperature, to ferment for four days. Stir the mixture daily. Mold will form on the top of the mass.
Pour off the pulp, unusable seed and mold that has floated to the top of the mixture. The usable seeds will sink to the bottom. Scoop them out of the container and spread them out on a sheet of paper towel to dry.
Place the dry seeds in the baby food jar and seal the lid. Place it in the freezer for 2 days to kill any pathogens. Then store the jar in the refrigerator until you are ready to plant the seeds in spring.