"While the many techniques and methodologies of seed saving could fill several books, seed saving is rather simple for the most part. To produce seeds with the highest germination rates, plants need to dry down completely and become "brown"—where all the energy from the plant has gone into the seed, and the remaining portion of the plant is mostly just standing carbon, ready to compost. All the sugars and nutrients were focused into the seed’s production. The seeds, at this point, may still need to be dried down further before storing which can be done in the sun, inside, or very carefully in a dehydrator (and then only for large, dense seeds like beans and corn).
Seeds are kept in a cool, dark place for the next season, or in the refrigerator or freezer for years. Seeds that are centuries and even millennia old yet still viable are being found on archeological sites and being grown out at universities continuously, proving how amazing seeds are!
Some plants such as cucumbers and melons need to be isolated from other members of their botanical family—they cross! To keep plants true, hand pollinate, plan and time accordingly, and use physical barriers like screens or paper bags" (The Permaculture Student 2, p. 190-191).