I've never made them myself, but have been wanting to.
Here's what my source says to do:
"Pick the green beans when young and string them on heavy thread, like long beads, one after the other. Hang the lines in a sunny place to dry thoroughly. It may take as long as a month or even two. When dried, store in baskets for winter use. To use, wash the beans wel. and soak 2 cups dried beans in 2 quarts of water for an hour or so. Add 1/4 pound diced salt pork, salt and pepper. Bring to a boil and reduce heat. Stir, then simmer very gently for at least 3 hours or until beans are tender, adding boiling water if needed to keep them from burning. Manna: Foods of the Frontier" From Reader's Digest Country Ways: A Celebration of Rural Life. p 141
Hope this helps!
Tanya