
It's almost black walnut time in north Georgia. I've had a few nuts dropping for a while, probably due to drought, but the main drop starts about now and lasts for a few weeks. The ones that have dropped already probably are not mature. The ones that dropped in July definitely were not ready.
Nuts are harvested when they hit the ground. They're covered in a spicy-scented thick green husk, which is removed to reveal the nut itself. English walnuts, which are the more familiar variety, also have husks but they split apart somewhat easier.

The first nuts I processed I used a hammer on; I just carefully whacked at them using the driveway as a backstop. It worked ok, but was very inefficient. I damaged a lot of the nutmeat and probably spent upwards of 20 minutes per nut. That was just a demonstration project to assess the viability of the nuts. I soon decided that I did want to harvest my nuts and went searching for a better nutcracker.
I soon found one:
It's available at: http://www.masternutcracker.com/. The website is not the most user-friendly, but there's a form that can be printed out and you can e-mail in advance with any questions you may have. There is at least one reseller I know of, and may be others: http://855sheller.com/. The tool does a great job and can crack the nuts with ease (it works on other tough nuts, like hickory). There do appear to be some knockoffs available for sale from a lot of outlets; they do not seem to function as well: Master vs. copy. I've ordered from the Master site twice and have had very good experiences.
855 Sheller also sells other crackers, including one that can be used with a drill to process nuts quickly--they have posted videos on their YouTube site showing it in action. If I had more trees, I would probably have to do something other than crack them one at a time, but I don't get a very big harvest. It's more of a novelty for me at present. Maybe someday the trees will be producing more, and I have just planted a new improved cultivar tree (Thomas Myers) that is advertised to yield big nuts with thin shells (I think one of my existing trees is something like this, or another big variety; the nuts run 20 to the pound).
A much easier option is just to buy them; the little packages at the grocery store are expensive but chopped black walnuts can be had from Amazon and other places relatively cheaply thanks to the business model of Hammon's, a Missouri company that buys nuts from small-scale harvesters who pick the wild nuts up off the ground. There do not appear to be very many commercial producers. The trees themselves are more often grown for their timber value, which can be substantial.