For those with lots of walnuts (also works some on pecans but better on walnuts) the old timey trick was to find a good sized knothole in a piece of wood that was just slightly smaller than the average nut diameter. You position the nut in the knothole and whack it down with something, personally I like a heavy shoe! Good hand size for me, good whack sound. You can also use a hammer.
In these modern days, maybe you could drill a hole in a board (I'd say at least 1" thick) that's the right size for your crop. Different years you might need different holes.
Typically you sit down, put a bowl in your lap/between your legs and the board across your lap. Yes, shells will go flying and there will be clean up both in the bowl and the general area. Good thing to do in an old wooden rocker on the porch. Watch those shell shards, they can be sharp!