A lot, unfortunately. They are a lot of work, to get the actual walnut fruit. Most walnuts you get at the market are from the English Walnut tree. Those are much easier to open.
Black walnuts are hard as feck. Literally. You gather them off the ground. The hulls are green in the tree and when they fall. You gather them when they fall. Remove hull when it is yellowish-green and you can leave an indent when you push on it. Then the hull will come off easily.
After removing hull, you have to wash the nuts in pails, vigorously agitaing the water/nut mixture to remove all debris. Usually need to do this 2-3 times per batch. Then spread nuts(still in shell) out on a screen to dry for a few days(away from squirrels, they LOVE them). Then put them in a cool, dry place to 'cure' for 4-6 weeks minimum.
Then, try and crack the shell open without crushing it to smithereens. The shells are HARD. Preferred method is to use a vice. I kid you not.
It's time consuming, but so worth it. They are much stronger flavor than the store walnuts(English variety).
Oh yeah, and the hulls dye your hands and clothes black if you aren't wearing gloves. Always wear gloves. And Black Walnut is toxic to some animals and plants. Can't dispose of hulls and tainted water in any stream or lake, will kill the fish. Can't put it in the compost or garden, will kill the plants. Earthworms apparently don't do well with it either.
But they are oh-so-good. Especially, in ice cream, cookies, cakes, etc. 😍