From the Yahoo! Group SCA-Herbalist:

Gather the acorns and shell them.  Our ancestors would put the acorns in a basket in a fast moving stream and leave them for a few days.  This allowed the tannins to leach out effortlessly.  Most of us don’t have a stream handy to use.  Cover the acorns with water and boil.  The water will turn dark from tannins.  Throw out this water and repeat the process as many times as necessary to get out all the tannins.  (This will vary by species.)  They say that you’ll usually need to boil the acorns 2-3 hours – changing the water several times in the process.  The batch I tried took
more like 4.  The nutmeats are then allowed to dry for storage (can be accomplished by heating up the oven and then turning it off then putting in the nut spread out on a cookie sheet.)  Or you can slow roast them.  They don’t have to be roasted unless you a) plan to make flour with them (I’ve never tried) or b) want to in order to compare flavors.

It’s a lot of work.  But they are high in protein.  Good luck, Avacyn

Maybe someday I’ll get to live near that stream, and find acorns…there’s also:

If you are actually going to try to eat them, please make sure they are
from white oaks(rounded lobes) rather than red oaks (pointed lobes).
White oak acorns develop in one year, so they don’t accumulate as many
tannins as the red oaks (take 2 years), so they’re not quite as bitter.
If I were going to fix them, I’d probably shell them, boil them (maybe
2x changing the water in between), then roast them. I’d probably freeze them afterward if I wasn’t going to use them right away, so that the
oils didn’t go rancid.

Good luck – let us know of your results!  Ceit