To save the seeds of a common bean (Phaseolus vulgaris) variety, simply let the pods mature fully on the plant. Harvest dry pods and shell out the beans which if not eaten are seeds! Take a handful of the soft newly shelled beans and give them a little shake - note the sound they make as they bump against each other. Allow the shelled bean seeds to cure in a warm, well-ventilated location, occasionally shaking a handful. As the seeds dry down you will notice a change in their bumping together sound as their seed coats harden. In a few weeks the sound stops changing and the seed coats are hardened enough for storage. Store your labeled container of seeds in a cool, dry location.
The flowers of common beans are for the most part self-pollinating, their design is such that pollination occurs before the flower is even open (the biological term for this is cleistogamous.) As a result, the chances of cross-pollination between varieties is low. To increase the likelihood the seeds you save from a common bean variety are true-to-type, separate varieties by at least 20 feet (6 meters.) Common bean varieties belonging to Phaseolus vulgaris will not cross-pollinate with other bean groups.