How is a windmill used as a mill for grain?

The old ones (also applies to water wheels) were geared. The rotation of the blade is geared to turn a shaft that is attached to a grinding stone. This rotates in a separate stone basin. Corn is added in one spot, ground by the grinding stone and the flour works its way out the bottom

They right angle gears to alter the direction of the force. Considering this mechanism was designed thousands of years ago, pretty clever!

They use gears to turn a large stone that would grind wheat into flour.

The same way a water wheel gets the job done.