Normally in Bon Ngondro practice there are three essence mantras, the Dharmakaya mantra, the Sambogakaya mantra, and the Nirmanakaya mantra. But Dru Gyalwa Yungdrung Rinpoche only mentions the DU TRI SU mantra, the Nirmanakaya mantra.
Du Tri Su Mantra Meditation
Sit in the five-fold posture. Visualize in the sky in front of you, your root lama in the form of Shenla Okar. In front of your root lama, visualize the inner and outer yidam deities, the Buddhas of the ten directions, Rigdzin awareness holders, and Yungdrung Senpas (Bodhisattvas), and the lineage lamas of the sutra, tantra, and dzogchen. Around them are the khandromas, and protectors – both wisdom protectors and worldly protectors. All are gathered like clouds in the sky in front and around your root lama.
Next, utter a long HUNG syllable and visualize countless light rays emanating from the HUNG in all directions. The light rays fill the sky in every direction. Through the power of the blessing of the dynamic energy of the rays of light, all sentient beings transform into rainbow light.
Then imagine all these rainbow lights from all directions return and dissolve into the essence of your consciousness.
Then the light from the HUNG goes out again in every direction. Through the power of the blessing of the dynamic energy of the rays of light, the entire external world transforms into rainbow light. All of this outer world of rainbow light from all directions returns and dissolves into the essence of your consciousness.
Then exclaim “Phet!” and imagine your body and consciousness becoming bigger and bigger until they are as big as the entire sky. At this point, your body and consciousness are as big as space itself. Then your body transforms into countless pure realms filled with Nirmanakaya Buddhas. In these pure realms, countless sentient beings of the three realms abides – desire realm, form realm, and formless realm.
Then reflect on the suffering of each being’s karma and through that generate profound compassion for those sentient beings.
Three Purification Practices
First, from the heart of the Buddhas visualized above us as we did before, the fire of primordial wisdom thunders with countless flashing sparks like lightning which instantly burns away the karmic imprints and obscurations of all sentient beings. This is the first purification of the blessing of fire of primordial wisdom of the generated object of refuge.
Second, the winds of primordial wisdom roars, moving like a shooting star blowing through all the pure realms. This wind of primordial wisdom blows away all the karmic imprints and obscurations of all sentient beings. This is the second purification through the wind of primordial wisdom.
Third, the water of primordial wisdom, moving like a shooting arrow and cleansing the karmic imprints and obscuration of all sentient beings.
After that, from the heart of the Buddhas (the objects of refuge), small and large seed syllables “Ah” arise filling the sky and dissolve into the crown of each sentient being. Then all sentient beings transform into the nature of Kuntu Zangpo.
Then, light emanated from those sentient beings whose nature is Kuntu Zangpo and all all pure realms become great celestial palaces of the Buddhas and wisdom yidams.
“Pure Realms” here means the place of all sentient beings, which include all the six realms and not just the Blissful Pure Realms of the Buddhas.
All the sentient beings’ activities become the same as the activities of the enlightened Buddhas. All the sentient beings’ minds become the same as the Buddha's profound unfabricated enlightened mind. All the sentient beings’ sounds resound with the sound of the Du Tri Su mantra.
This completes the contemplation stage of the Du Tri Su mantra.
Next, stop talking and recite the mantra:
A KAR - A ME - DU TRI - SU - NAK PO - SHI SHI - MAL MAL - SOHA
You can recite 100, 1,000, 10,000, or 100,000 times. When you finish reciting the mantra, recite the praise of the mantra below and rest in the completion stage.
ཨ་དཀར་ཨ་རྨད་དུ་ཏྲི་སུ་ནག་པོ་ཞི་ཞི་ལམ་ལམ་སྭཱཧཱ།
༼ AKAR AME DU TRI SU NAK PO SHI SHI MAL MAL SOHA ༽
A KAR, the nature of the mind is completely pure.
A MED, primordial wisdom is clear light.
DU TRI SU, the liberator Buddhas of the three lower realms.
NAG PO, the purification of negative karma and obscurations.
ZHI ZHI, the pacification of the suffering of cyclic existence.
MAL MAL, the blissful mind.
SO HA, the destruction of wrong views.
By this mantra that pacifies suffering,
From emptying the depths of the lower realms of cyclic existence,
May all beings attain the ultimate reality of Buddhahood!