The basic controls within the AutTune interface are:
Retune Speed = 0
Choose the appropriate scale
There are a few possible variations in approach, depending on whether you choose to use Automatic or Graphical Mode.
Automatic Mode
Begin with the Retune Speed at 0.
Set the Key/Scale to the corresponding performance.
This is where you can become crafty and explore the possibilities.
If you are uncertain of the key or scale of the song, try randomly choosing various scales. You may arbitrarily stumble upon something unique.
If you begin with a simple Major or Minor scale you can certainly find something within that scale that will work, though choosing the chromatic scale will allow you any note choice, but won't typically activate the effect.
I sometimes find that limiting the note choices will cause AutoTune to react erratically and thus create an even more exaggerated effect.
Set the Retune Speed at 1 or 2 or a bit slower. This will allow slight pitch variations and slightly less instant note transitions. Faster Retune Speeds will cause the note to be faster and create instant effect.
Graphical Mode
Begin by setting the Graphical Mode's Retune Speed to 0.
Switch to Auto Mode and choose the Chromatic Scale.
Return to Graphical Mode and track the vocal so that it can be analyzed and "read" into the system.
Select the Line Tool and choose the Snap To Note button.
Draw lines for each note you want in the melody.
To ensure instantaneous activity between notes, "butt-splice" each note against its adjacent notes.
If you're unhappy with the result, select the Arrow Tool and experiment with changing the pitch or length of individual lines.
Another option that I have found works really well is to have the vocalist perform the track live with AutoTune active.
This will allow the vocalist to manipulate the vocal inflections to activate the effect with ease. It will paradoxically create a more organic result, since it will become a performance piece and not altered in post-production.