Articulation of tongue in each vowel.
2) When you can pronounce "ah", take in air and say "aw", take in air again and say "ay", then "ee" and "oo". Try repeatedly to pronounce vowels in succession: "ah, aw ay ee, oo."
Try to pronounce vowels clearly adjusting the form of your mouth to suit the exact pronunciation of the successive vowels.
3) Then practice the pronunciation of other diphthongs: "oh", "I", "oy", "ow".
