First of all, make sure you have a High-Gain setting dialed in on your amp, and put your
pickup switch to the
bridge pickup. Low-output pickups are going to make them harder to hear. Now, to do the scream, you need to hit an Artificial (or Pinch) Harmonic on your open-G string. With your whammy bar, push it down, then pull up and you'll get them screaming pretty good.