The Mantra of Rest is derived from breath counting techniques that are taught in many schools of meditation, including Buddhism and Taoism. Around those counting techniques I have placed the verbal formula “It is by quiet breath I set my mind at rest.” This verbal formula provides a counterpoint to “It is by will alone I set my mind in motion.” Treat them both as a yin yang pair: to be an effective mentat, you must train techniques that help your mind to rest and techniques that help your mind to think.
Breath counting techniques are meant to focus the body and calm the mind. Unsurprisingly, these techniques also help you focus your mind and calm your body. With both your body and mind calm and relaxed, you are ready to focus on the task or problem at hand.
Begin the Mantra of Rest by reciting its opening sentence. Then proceed to count your breaths. When you lose count, repeat the opening sentence and start again.
For more context and training concerning these breath counting techniques, consult any source that teaches basic meditation, such as Taoist Meditation by Thomas Cleary or Turning the Mind Into an Ally by Sakyong Mipham.