I am of the opinion that mantras must be “acquired”.

My favorite form is to acquire it from someone who has already mastered the mantra.

When that is not available, the second favorite, is to acquire it from someone who acquired it from someone who mastered the mantra.

And if that is not available either, as a last resort, it is to break the mantra down into syllables and use intuition and experience with other mantras to construct the pronunciation, while getting into the rhythm through experimentation.

With that disclaimer, the following video is perhaps the best video I have seen – in a long period of time – regarding mantras:

(Many thanks to the teacher)