Bean seeds are easy to grow unless the autumn frosts arrive early or continual rains cause the pods and seeds to mold.
They are annual and self-fertilizing but cross-fertilization between other sorts does happen. Do not sow 2 sorts beside each other. At least 50 m is necessary to get absolutely pure seeds of each sort. Normally it is enough to have one other crop between the sorts. To keep the genetic variation in a sort you should grow at least 25 plants for seed, preferably more.
Grow the beans as usual but remove deviating plants carefully (the late bloomers, plants sending out runners, differently shaped leaves and the ones which are tainted by some disease).
The seeds are ripe when the plants begin to wither and lose their leaves.  Remove some of the youngest pods if you want faster ripening. Light frost doesn"™t harm ripe seeds but does harm unripe ones. Pull out the whole plants and allow them to continue ripening indoors if there is risk of harmful frost or rain damage.  
In a small plot one can pick the pods by hand and let them continue drying indoors before threshing. If there is a large amount of pods pull all the plants out and hang them up to continue drying outdoors or indoors depending on the weather. A shower or two do no damage.
The pods must be really dry before threshing either by hand, flail or threshing mill. Other methods are hitting the pods against the wall of a barrel; even dancing on a sack of ripe pods is an excellent method. Some pods hold on to their seeds hard and must be broken up by hand.
Remove large pieces of rubbish and then sieve and winnow away the rest. After threshing with a flail you can just rake the rubbish away.
Before the dry beans are put in paper bags they should be looked over. All beans that have the wrong colour, look unripe or are tainted should be thrown away.
Well-ripened beans keep their fertility for 4-5 years in a dry and airy space, packed in paper bags with names and dates on them.