Ok, so after talking with Mark in another thread, it dawned on me that there are probably no small number of folks here who have no idea how to prepare corn, much less other grains, for grinding.
I covered how to "crack" corn out of it's husk in that thread, but I'll cover how to actually make potash here.
Potash, essentially, is the first base that man discovered. It's what's left when you burn word and then soak the ashes in water. Welcome to the wonderful world of lye (also handy for soap making). Caveman to Chemistry covers this in depth.
Here's the short and sweet of it:
burn a big pile of wood to ashes. MAKE SURE THERE ARE NO PLASTICS OR OTHER CONTAINMENTS IN THIS WOOD OR YOU WILL DIE. Then scoop up the ashes and put them in a 5 gallon bucket of water. Eventually, the water soluable potassium and sodium salts will separate and the junk will float to the bottom. Pour this through a mesh strainer and use the limewater to crack corn kernels for grinding. You can also use the the lye for making your own soap and the ashes also make good fertilizer.
Lye/limewater is a deadly poison and will kill you, your children, and your little dog too. Keep them out of it, dispose of it away from your sources of drinking water, and make sure nobody drinks it.