AH-EH-EE-OH-OO-YOW

Repeat each syllable nice and long on the same note. Make the final YOW an octave higher and give it a bit of a punch. Make sure you hit that high note dead on. Repeat with the next note up the scale. Make very sure your sound is coming from the front of your mouth - vowels like this are the easiest sounds to lose in the back of your throat.