It is usually called the “Jesus Prayer.” This is how it goes: Lord Jesus, Son of God, have mercy on me, a sinner.
This is united with one’s breathing in and out. After some time, the prayer bevomes so united with one’s breathing, that the breathing itself becomes the prayer all by itself. It is good to find a quiet place to begin with. You can use an Eastern rosary which they call a prayer rope. It’s usually made of yarn. Or you can just pray it without a rope.
I suggest that you read the book, “The Way of the Pilgrim.” It’s a classic.
This link on Eastern Catholicism might be helpful.
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