Place the rye berries in a blender or seed grinder and grind.
Sieve the powder to collect larger bits. Grind them again. You may need to do this a third time, but not usually.
If using a blender then let it run for 40-50 seconds non-stop then sieve and re-blend the larger pieces. When using a blender you will need at least 2-3 cups of the grain, to properly work and cover the blade.
Transfer to an airtight jar. Whole grain flour won't last as long as version without the whole grain. Keep it in an airtight container ( very important!) in the fridge for 3-4 months. You can also keep it in the freezer for around 6 months.The cold air will slow down the oxidization process, stopping the four from turning rancid too soon. You'll know when it has gone rancid as it will smell unpleasant.
Recipe on Alphafoodie: https://www.alphafoodie.com/how-to-make-rye-flour-pumpernickel-flour/