The 'ow sound' /aʊ/ is a two-sound vowel that ends in a brief 'w sound' /w/.
Part 1: The sound begins with the jaw open and the body of the tongue in a low position in the mouth. The bottom teeth can be felt along the sides of the tongue.
Part 2: Then, as the jaw closes, the lips close into a small circle similar to the position of a 'w sound' /w/. The body of the tongue moves upward until the tongue is near the back of the hard palate.