I imagine the basic process hasn't changed much in 10,000 years.

One of the items we found on my grandparent's farm was a hand-made flail, a long pole attached to a short pole with a piece of heavy leather as the hinge. This design, and its variants for different grains, are older than the pyramids.

I've threshed grain with a combine (but not by hand), and the one thing I remember vividly is the importance of the moisture level. The grain stalks have to be thorougly cured (allowed to dry) before you start. Otherwise it's nearly impossible to separate the grain from the stalks and the kernel from the hull (chaff). After that, be aware of the ambient humidity. Even the big operators with half-million-dollar machines have to shut down when the dew starts to fall.

My 2¢.