The green outer shell will normally split and often the nut will fall off the tree of its own accord, at this point especially if there's a heavy wind. However, from around mid September onwards they can be picked off the tree with the green husk on and left to ripen indoors until the husk turns black and shrivels around the nut.. However, you may then find you need a sharp knife to scrape the husk off. If you're thinking of making pickled walnuts from them the husks need to be very young, green , fresh and well pricked. . I "scrumped" about 3 lbs of walnuts from a wild tree a couple of years ago to pickle them. Obviously I did something wrong as when we came to eat them, the walnut shells were hard as a rock and totally inedible so it was a waste of effort.