Vijnana Bhairava Tantra · Dharana 7 · Verse 30
Twelve letters, then silence
Work through the twelve vowel sounds of Sanskrit, one after another, or through any simple vowel sounds. First speak each one aloud. Then feel only the vibration it leaves in the body. Then sit in the silence after the vibration. Then leave even that silence aside. What you are after is what remains when the sound is gone.
Older line
Contemplate in turn the twelve centers, each marked by one of the twelve vowels: first in a gross form, then a subtle one, then the supreme. Letting go of each in turn, at the end one is Shiva.