AT MASTER TU'S KIM SON MONASTERY WITH JANEY TRINKLE
After hearing a mantra from somebody who has the transmission, one learns and memorizes it. Short mantras like OM MANI PADME HUM or NAMO AMIDA BUTSU are easy but to become skilled at mantras, take something longer, at least 20 syllables, for instance the mantra of FUDO MYO-O or the 100-syllable mantra of VAJRASATTVA or the DAI HI SHIN DARANI (the Great Compassionate Heart Dharani).
Once it is memorized, begin repetitions on a daily basis. Every syllable needs to be clearly pronounced. However, sometimes you may experience getting tongue-tied & mush-mouthed, thus garbling the mantra. In such cases, employ a well-known technique used by radio announcers-in-training:
First, take a moutful of 10 marbles and practice reciting the mantra this way until it becomes smooth and clear. For the next session, remove one marble and practice with a mouthful of nine. Then eight, and so on until the last marble is removed. When you have lost all your marbles, then you are certified as a Mantra Yogin!