A vice and a pair of pliers works good, but so does a hammer on a sidewalk. They actually make a nut cracker for them.
Here is some info from another site:
The first nuts dropped by the BW trees are often "blanks" - unfilled or poorly filled nuts, but when the main crop begins dropping, they're usually quite well-filled.
You really need to remove the husk material before it gets black and mushy, as it can penetrate the nutshell and cause discoloration of the nutmeat as well as conferring a somewhat bitter flavor to the kernel.
I dump freshly collected nuts in my gravel driveway and drive over them for a few days to loosen the husks - you can then manually remove remaining husk material - but wear gloves if you don't want your hands/fingers stained. Or, you can roll the nuts under your booted foot on a hard surface to remove the husks, run them through an old hand-cranked corn-sheller, or pound them through an appropriately-sized hole in a sturdy board.
Once you've removed the bulk of the husk material, place your nuts in a bucket of water and stir briskly with a stick, changing the water until it no longer comes off looking like strong black coffee. At this time, you can also 'float' test your nuts - most incompletely-filled nuts will float, and should be discarded, while well-filled nuts will sink. Crack a few floaters, though, to make sure they're no good - some really thin-shelled selections tend to float, and you don't want to throw away your best nuts!
Once your nuts are clean, spread them in a cool, dry place to 'cure' for at least a couple of weeks - straight off the tree, they're not tasty.
There are a number of good walnut crackers on the market, but I just use a simple bench-mounted vise(though I'm planning to buy a "Mr. Hick'ry" Nutcracker this year). If you'll soak your nuts in warm water for an hour or so prior to beginning a cracking session, you'll find that the shell will tend to buckle and split rather than 'exploding' once you reach 'critical pressure' - you'll get more intact quarters and fewer tiny pieces, as well as fewer 'extra-crunchy surprises"(chunks of shell). A pair of diagonal wire-cutting pliers and a nut pick are handy for snipping shell here and there to release quarters and larger pieces.
