Set the knobs to to auto-tune levels. Though you can play with these knobs as much as you want to get a unique sound, use the following settings to get the "classic" autotune sound:

    MIN FREQ: 80 hz
    MAX FREQ: 2000 hz
    GATE: -40 db
    SPEED: Between 1 (choppy vocals) and 10 (smoother vocals)
    THRESHOLD: 100 cents
    AMOUNT: 100%
    ATTACK: 1 ms
    RELEASE: 61 ms