Acorns are a nutritious, versatile nut, but they are loaded with tannins, which are bitter substances that make the nut meats inedible and can cause digestive issues. They must be removed before eating or cooking with any acorn.
Various processes have been recommended for removing tannins, all of them long and multi-step. One of the easier is the 2-pot method described here.
First sort out and discard any green, worm-infested or rotting acorns. As a final part of the sorting, place the acorns in a large bowl and fill it with water. Any acorns that float likely have gone bad and should be tossed.
After you’re relatively certain that you’ve removed all the bad acorns, crack open the remaining nuts and remove the meats from the shells.
With just the nut meats in hand, get 2 pots of water boiling.
Toss the nut meats into one of the pots and continue the boil until the water looks like strong tea.
Strain the nut meats through a colander and drop them into the second pot of boiling water.
Rinse the first pot, fill it with fresh water and get it to boiling once again.
When the water in the second pot looks like strong tea, strain the nut meats through the colander and drop them back into first pot, which is boiling once again.
Repeat the above process until the water boils clear. Do not allow the acorns to cool until the end of the process.
Cooling before all tannins have been removed can allow some setting of the bitter materials into the nut meats.
At that point the acorn meats are ready for use.