Acorn flour

Gather acorns in the Autumn. They can be stored to make flour all year round.

Here is another article describing how to make acorn flour, and including pictures.

Some more information about traditional handling and uses of acorns can be found here.

Ingredients

Dry acorns in the oven using the pilot light or very low heat. This will not just dry them, it will also kill off any insects.
Crack the shells off, and then soak the shelled acorns in water for several days to two weeks, changing the water at least twice per day to remove the tannin. This process greatly differs in time due to the type of oaks. Tannin is actually poisionous, so be very careful when making this flour. The process might need to repeated. If you are not sure if the tannin has gone make sure that you ask someone who knows what they are doing for advice, or look at the nut factory website.
When the acorns are no longer bitter (just bite into them to test), grind them while wet through a meat grinder - if you do not have a meat grinder then a good blender will do, or even a mortar and pestle if you have a lot of time!
Then place the coarse meal into pans to dry in the sun or oven (again on a low heat).
When dry they can be stored in large jars in the cupboard. The meal/flour can then used in place of wheat flour in recipes.