You can read lots of advice about making acorn flour by searching online. Kelly and Erik from Homegrown Evolution have an excellent section on it in their book, The Urban Homestead, which, if you haven't bought already you should right now. I made a few batches this time last year, before the book came out and what worked best is pretty much exactly what they describe. I wish I had the book then.
1. Collect your acorns.
Avoid anything that has a damaged shell, especially a dark hole or small circular scar on it about the diameter of a pencil lead. Those acorns have worms in them and are no good.
2. Shell the acorns.
This was the most time consuming part for me. I learned a trick for shelling acorns toward the end of last year's season: When dry, many will open themselves. I'm not sure if this is true of other types of acorns but our Live Oaks pop right open when dried out. If you wait for them to dry out, make sure that they get plenty of air, you don't want them to rot. If you have a dehydrator, you may consider speeding the process. Keeping them in the oven may work too if it has a pilot light.
3. Make acorn mush.
Put them up in a blender with some water. Don't be stingy with the water, you'll be rinsing them out several times before your flour is ready. Think acorn smoothie.
4. Rinse out the tannins.
Acorns contain tannic acid which is bitter, and not good for your kidneys (or iron absorption) . The good news is that it is water soluble and easy to remove. Drape a cotton dishcloth over a deep bowl, pour in your mush and rinse the it with warm water. Wring out the mush by bringing the corners of the towel together and twisting. Taste the mush, if bitter, repeat. I have heard that you don't need to make mush before rinsing the tannins out. I tried this method and my acorns were still bitter after several weeks of changing the water every day. I also tried using boiling water to rinse the mush but warm water from the tap seemed to be the best balance between water usage, gas usage and time - at least for me.
The waste water is good for plants.
5. Dry out the mush.
Next spread the mush out on a cookie sheet and either leave it in the sun on a hot dry day, put it in your dehydrator, or put it in your oven after you baking some cookies or something and let the residual heat do the job. Stir the mush occasionally to speed the process. If it clumps up and looks like ground beef it is probably going well.
Once the flour is dried out it may be a little coarse. You can put it in a cleaned out coffee grinder to get a finer texture. A good food processor also works and I am pretty sure they make attachable gadgets for mixers that really mill the flour if you get completely obsessed.
Our favorite use is acorn pancakes. Just mix the acorn flour 1/2 and 1/2 with wheat or other flour from your favorite recipe. I love the acorn flavor - slightly nutty, very hearty. If you make your own, let us know how it went.