They need to be allowed to dry while still on the vine. After the pods are thoroughly dried, they will be a pale beige in color with a papery feel, and then you can pick them off (sometimes it's easier to snip). Spread the pods onto newspaper, because even though they *look* dry, there is still moisture. After a couple weeks, you can shell the seeds. Store the seeds in an airtight container in a cool, dark place (I put mine in the freezer).

BTW: a) remember that hybrid beans will not come true; the seeds' plants will exhibit the characteristics of the 'grandparents' not the parent plants. b) Saving non-hybrid seed is an opportunity to improve the strain, so save the best pods according to your preference (biggest or smallest, fattest or thinnest, longest, earliest, latest, and/or yummiest from the healthiest plants). Never bother saving spoiled or diseased pods. c) Pods which seem too dry to eat as green but not dry enough for seed-saving can be shelled; add the dry-ish beans to a soup or simmer until tender, drain, stir in a tiny bit of butter and then sprinkle with marjoram for a tasty side dish.