Here is how you pronounce Om or Aum.
The 'O' or 'AU' is said externally and the 'M' is allowed to reverberate internally.
The first half of 'O' or 'A' starts in the chest at the anahata chakra, moves upward with the second half of 'O' or 'U' to the vishuddha chakra in the throat and ends with 'M' vibrating the ajna and sahasra chakras in the head.
The way one sits during meditation is most important as well. Sit with spine/back straight, (to facilitate the flow of Kundalini energy up your spine, the twin male/female energies that wind their way up and around the spine) and with your feet either planted firmly on the ground or your ankles crossed, whatever is most comfortable for you, gently close your eyes. Gaze in upon your brow or third-eye area. Slowly become aware of your breath or heart beat: up and down, in and out, but don't force your breath, merely observe it. Place your tongue to the roof of your mouth just behind your teeth, this is one of two Hun Yin positions, the other being at your rectum, where you can tighten your anal muscles. Blocking both Hun Yin positions during meditation helps build up your Chi/Ki/Prana (Primal Energy) which is stored in your belly, also called the Hara. This encourages longevity, and overall good health.
When doing healings, (Reiki, Ling Chi, Qi Gong, Prana) you may also add this "original" Chi/Ki/Prana stored in your belly: inhale deeply (into your belly, which expands) with tongue to the roof of your mouth/upper palette, then exhale, (Your belly contracts) dropping your tongue to the lower palette, exhaling through your mouth. This pushes the stored/"original Chi" in your belly out to make for a stronger and more effective healing. Qi Gong healing is often accomplished this way. As with Pranic breathing, you can inhale into your belly six beats, hold for three beats, then exhale from your belly to the count of six over and over during the healing session. Remember to include your tongue/Hun Yin positions as explained.
Of course, while meditating, you can also do this Rythmic/Prana breathing, (six-three-six-three, etc.) including holding your tongue to the roof of your mouth, but breathing in and out through your nose. (Dropping the tongue, etc. is only used during healings, but is not wrong if you do it during sitting meditation) As long as you sit in quiet contemplation, palms facing up with the back of your hands resting either in your lap or on each thigh, or even sitting in Lotus/Half Lotus position, eyelids shut, with you gazing upward towards the brow/third-eye area. Sitting on your knees with your heels touching your buttocks during meditation, each hand overlapping on your Heart Chakra is a very traditional Japanese Buddhist Meditation stance/position too! Still another traditional Japanese Zen Meditation position/method is to sit in Lotus position, and with eyes half opened, gaze on the floor approximately three-to-six feet in front of you, following your breathing. The reasons for the half-opened eyes (according to myth/legend) is so that you don't let too much of your own light out, nor too much in.
Experiment with all of these; as long as you sit comfortably following your breath, you can't go wrong. Add a Mantra in, ("OM" or otherwise) and you've got all spiritual bases covered! Enjoy.
For more types of meditation, go here: https://www.drmichaellikey.com/primordial-sound-meditation.html