Saving bean seeds
I have been gardening and farming all my long life and still have problems saving my runner beans for seed. If I pick them when they are still green and either leave them in the pod or take them out, they either rot or shrivel. If I leave them later, hoping they will ripen and harden and we get a frost, that seems to finish them. If I am lucky enough to find some that have withered and gone brown, then I am in luck, but there are not many of those.
So can you please tell me what I should do?
I suggest that you earmark a couple of plants at the beginning of the season for seed saving. Don't pick ANY pods from them to eat - just pick the crisp brown pods at the end of the season. Don't feed them, or water them unless it is very dry - as this can encourage leafy growth rather than pod development. There is no point in picking green pods as the seeds are not mature enough at this stage.
Did you know you can save the roots, overwinter in a frost-free place, and replant next year? Runner beans are perennial, but are frost sensitive, so die back in our climate. However, if the roots are dug up and kept in suitable conditions, the plants often get away early and crop faster. If you grow a lot of beans, this may not be a practical option, but you could try it with one or two plants perhaps. Store the roots in a frost-free place, buried in slightly moist sand or leafmould, or something similar.