Aleister Crowley has two books that provide details on yoga, and specifically, breathing techniques. Eight Lectures on Yoga details the basics of yoga, and Magick: Liber ABA (Book IV) provides detailed instructions for 11 different practices of yogic breathing and meditation. The exercises in this series are derived from Book IV.
Beginners to yoga and meditation should start with this first practice, and gradually add the others. Here’s how to do the first practice:
Concentrate entirely on the act of breathing. Mentally repeat to yourself, “the breath flows in,” and “the breath flows out.” Keep a record of your results: How long you practiced (about 20 minutes per session should be the minimum); your level of concentration; the number and type of breaks in concentration; any physical, mental, and emotional states; the date and time; and any perceptions after your practice.
Practiced diligently, Crowley says this technique may induce Samadhi, the eighth and final limb described by Patañjali in his Yoga Sutras.
Julius Evola mentions this technique in The Doctrine of Awakening: The Attainment of Self-Mastery According to the Earliest Buddhist Texts. He advises the student to breathe in deeply knowing, “I am breathing in deeply,” and while breathing out to know, “I am breathing out deeply.”
*Note: My guide to the second practice of yoga can be found here: Crowley on Yoga: Second Practice