Broad beans are annual and basically self-fertilizing ,however insects can transfer some pollen. Grow just one sort, or keep the different sorts apart, at least 500 m to get pure seeds of each sort. Sow the beans early and grow them as usual. Remove deviant plants before blooming, possibly top the plants once blooming has begun, to expedite fruiting and prevent attacks by aphids.
When the pod has grown leathery and black the seed is ripe. The seeds ripen unevenly and the pods should be picked by hand now and then in smaller cultivations and continue ripening indoors. Otherwise cut off the whole plants and bunch them for drying outdoors on drying racks.
The pods are threshed when they are completely dry, either by hand, flail or threshing mill. The big seeds are easy to clean. Look over the seeds and throw away all deviant ones, before storing them in paper bags.
Broad bean seeds keep fertile for 4-6 years.