Go to your nearest park or forest with oak trees in them and collect a bag full of acorns (or as much as you need)
Dry them in the oven using the pilot light (‘s’ on many gas cookers) 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. Generally, I soak the acorns for a few days to about two weeks, changing the water at least twice a day. This is to get rid of the tannin, the substance used to tan leather which also gives the acorns that bitter taste. This process greatly differs in time due to the type of oaks, and the American Oak is a lot less problematic than the UK oak as the acorns from UK Oaks contain more tannin. Tannin is actually poisonous, 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.
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. I have used a pestle and mortar in the past however I had a lot of time on my hands.
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.
Acorns were used in this way by the Tsaagi (Cherokee)