The positive post from the battery goes to one side of the Ford solenoid. The opposite post of the solenoid goes to the starter. You connect a piece of steel or copper from the Chevy starter solenoid where the battery connects to the post that had the "S" wire from the ignition switch. This effectively shorts out the Chevy solenoid and causes the engine to crank when power is applied through the Ford solenoid. The wire that connected to the Chevy solenoid now connects to the small post on the Ford solenoid.
Found a couple sites that explain it better.
http://www.maliburacing.com/starter_solenoid.html
http://www.454malibu.com/ford_solenoid.htm