If memory serves, the tribes around here soaked them in streams, ground them and sun dried the paste, all of which was designed to leach those tannins. Acorns were a mainstay food crop here in the Northeast, and I like the idea of getting them back into our diet (I like the idea of getting any native local food back in our diet!). I will do some homework and post my findings here...
I've messed with acorns as food before, mostly with white oak...though chestnut oak is allegedly even sweeter/lower in tannins, I never catch 'em before the squirrels do (which is a good sign, methinks)...I soaked them shelled in several changes of water and then boiled them in another few changes of water...the nuts get darker and darker as you go, and after I ground, dried, and ground them again, the resultant flour (and small container of coarser meal that resisted grinding) was dark brown and fairly aromatic/rich smelling. I've used it in place of cornmeal in cornbread recipes, both alone and mixed...the breads seem to tend toward dryness & density, but I never felt I made enough flour to test recipes thoroughly for fine-tuning. kinda want to use acorn flour and chunked up chestnuts, with some binder (maybe eggs?) to make nut cakes, fried fritter-style...