Using a Chotki (also known as a prayer rope or Komboskini) or simply on your fingers, recite the following prayer:

Lord, Jesus Christ,
Son of God,
Have mercy on me,
A sinner.

Try praying it this way:

Upon inhalation: “Lord, Jesus Christ, Son of God”
Upon exhalation: “Have mercy on me, a sinner.”

Or aligning the words with the beats of your heart.

Extended versions of this prayer can be found here and additional information has been presented beautifully here, here, here, and here.

You can also listen to variations of it here, here, and here.

Pray without ceasing.
1 Thessalonians 5:17

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