The mantra I am and Shri Om, do they have to be said exactly, is the main thing the letters, and the order doesn’t matter, such as Rash, or Maya, or mo, ma etc.
I understood from one of the lessons that one should say Shri Om, Om Shri, and not twice the same thing Shri Om Shri Om, is that correct,
So, in AYP everyone starts out with the same mantra, which is AYAM. This can also be written “I AM”, as it is the same sound in English.
There are then two different progression sequences for moving on to additional syllables for those who wish to do that. These two progressing sequences are described here
So, you would choose one of those. If you are not sure which to choose, I would suggest using the second. That lesson talks about why someone may choose to go with the progression sequence that uses less doubling-up of specific syllables.
The order is important. So, if the mantra you are using is SHREE OM SHREE OM AYAM AYAM NAMAH, then you would think the words in that order.
The pronunciation is also important. The pronunciation for all the syllables used in the AYP mantras can be listened to here. This is an audio of the second mantra enhancement from the first sequence progression.
Once you are meditating, the mantra, no matter how many syllables you are using, will often refine and become less clearly pronounced, and it is fine to let that happen.
In my Video Course on Deep Meditation the last week of the course (week five) looks at the process of adding these various additional syllables and at the two different progression sequences.
You just have to say the mantra silently and trust the process. The mantra may change in speed, clarity, refinement, etc. during a practice and from practice to practice. There is no need to monitor these subtle changes because this splits the focus between the practice and the monitoring of the practice. Mantra meditation is sometimes described as a one-point focus, so during the meditation we only do the mantra and nothing else.
Outside the meditation practice, it is fine to reflect on your practice.