Once that's done you run a special canvas belt around the hub of the windmill's blades and over the rimless wheel, now doing duty as a pulley. You start the tractor and place a block of concrete or a spare anvil on the brake for the wheel still on the ground, stick the tractor in gear and engage the clutch.
It's then a simple matter of slowly unjacking the tractor until adequate tension on the belt is achieved for the windmill's vanes to begin turning.