I put my black walnuts in a burlap bag and drive over them. Not pretty, but effective.
Here is Hank Shaw's method (i've just cut and pasted the important parts. If you want to read the whole article, it's here http://www.theatlantic.com/health/archive/2010/12/black-gold-the-pleasures-of-a-hard-to-crack-nut/68212/
First you need to harvest the walnuts at the correct stage of ripeness.For starters, walnuts will probably not be ripe where you live until October, so wait until then to begin. November should be fine, too, and you can pick up fallen nuts from the ground around the trees into December, but by then our Little Gray Friends the squirrels will have had at them.
So you're standing at a tree. You see all these forms of walnuts in front of you. Which to pick?
Green ones will most likely still be on the trees. Yes, you can collect them, but they have a surprise in store for you. The beige ones are rotting green ones—they are the hardest to work with, but the nut inside will still be fine. The black one at the bottom is how you will find most of your walnuts: It has its hull rotted and is pretty dry. Finally, if you've had lots of rain, you will find some nuts that will be pre-hulled, like the one under the half-shell. Pick only pre-hulled walnuts that feel heavy for their size, as they will dry out in the shell once hulled.
For the most part, you will need to hull your walnuts. Lots of people say you should just drive over them with a car, but this stains your driveway. Stain? Why yes. Black walnut juice stains like nothing else. And it will not come off with any amount of scrubbing. If you fail to wear gloves when you hull black walnuts, you will have the Black Hand of Death for several days.
Sam Thayer, in his book Nature's Garden, suggests stomping on the hulls in the field to get them off. This works, but incompletely in my experience with Northern California walnuts, juglans hindsii. So I sit outside on my porch with three buckets—one with walnuts in it, one for the soon-to-be-hulled walnuts, and one for the hulls. I then don gloves and use a pocketknife to hull the nuts by hand.
You want a relatively dull knife that you can slice with and not be in danger of it piercing your work gloves. The work can get a little slippery, especially with the green walnuts—remember the surprise? That's it. A half-hulled green walnut is slipperier than goose ^%$# on a doorknob.
This is why I prefer the walnuts with the fully rotted black hulls.
Beware when you are hulling walnuts outdoors. Our Little Gray Friends could be lurking anywhere, just waiting to steal your walnuts for themselves. I use a biological countermeasure to keep the squirrels away (see the photo below).
Once you have hulled your walnuts, your work has just begun. Now you must shell them.
This is the point at which you can kick back a bit. Hulled walnuts store well in the shell, and in fact crack better once they've dried for a few weeks.
Once you start shelling, however, you need to banish from your head all notions that you will be able to crack black walnuts and get those pretty perfect halves you can get with regular walnuts. Won't happen. Bits and pieces are the price of precious black walnut meats.
I crack mine with a hammer, on the concrete floor of my garage. Such force is necessary. I've never heard of a regular nutcracker fierce enough to break a black walnut, although some people in the Midwest, where the Eastern species lives, have created special black walnut shellers. Anyone ever use one? I'll buy one if they work well....
The key to the hammer technique is to use a terrycloth towel to cover the nuts, so the pieces don't fly all over the room. Use a towel you don't care about, as it will get holes. Smack the nut with enough force to break it, but not enough to pulverize the nut; after a few, you'll get the hang of it.
So now you have a bucket of cracked nuts. You're still not done! Now you need to gently remove the meats from the impossibly complex interior of the walnut. I find the best piece of equipment to do this is a stout pair of wire cutters and a nutpick. I use the wire cutters to clip the shells in key spots so larger pieces of walnut fall out. Again, after a few dozen nuts you'll begin to know where to clip. The nutpick's use is obvious. This is tedious work, people. Do it during commercials of your favorite TV show.
Let me tell you before you begin a black walnut adventure that you need to be patient. It took me probably six hours of work to get 15 ounces of nutmeats—although that includes hulling all of my nuts, not just the portion I cracked and picked. But all this work is worth it.
Just the aroma of black walnuts is payment for the effort: They smell toasted without actually being so. And I've already mentioned the flavor, which is so strong many recipes say just half the amount of black walnuts will fully replace the flavor of regular ones. I'm not so sure about that, but you can use a bit less. If you want.