Okay; here's the entry where I started. If you click on the "acorns" tag, you'll get all the rest (plus, more recently, unrelated posts). Basically:

You need to extract the acorn meat from the shell
Then grind them up coarsely
Then soak the ground-up nuts, changing the water each day, until the water starts seeming fairly clear (for me, when I used red-oak acorns, it was two weeks).
Then dry them in the oven
Then grind them finely
Then use in a cake! (this was the recipe I used for the cake)