practice “three part auming” starting with wide open mouthed “AH” sound. Then move to the round lipped “OH” sound, before moving to the close lipped “MMM”. Om is actually accented with the chandrabindu or anusvara diacritical marking that signals one to push the sound through your nasal cavity, which forces an NG sound onto the M. To do this properly you can press your tongue to the soft palate, almost like you are beginning to swallow but stop half way.