Nadi shodhana (sometimes called "alternate nostril breathing", often called "nerve purification" but literally meaning "channel" or even "tube" cleansing) was highly recommended by the "root guru" of the yoga lineages I teach, the "Father of Modern Yoga" TKV Krishnamacharya, seen above practicing nadi shodhana at age 87. This is a safe, effective pranayama practice for mental clarity, for calming the mind and body, for dealing with stress, anxiety, fear. It is also a preparatory practice for deeper and more challenging pranayama practices. From my own experience as a dedicated daily nadi shodhana practitioner of about 20 years, and from teaching it to many students over the past decade, it is the easiest, gentlest and most effective form of pranayama, accessible to all, when done correctly. Folks with high blood pressure and who are pregnant may do this practice without any retentions/holding of the breath.
-To practice nadi shodhana, sit comfortably, and ideally, relax your shoulders, soften your jaw, breath gently, and make a mudra with your right hand, called vishnu mudra.
(If Vishnu Mudra doesn't work, simply use the thumb and forefinger of the right hand, or alternatively, place the middle and index fingers of the right hand on your third eye/brow, and use the thumb and ring/pinky from that angle.)
-Breathe in a relaxed way, fully and deeply through both nostrils, for a few breaths.
-Then, block your right nostril with the right thumb (not at the "hole" but further up closer to the place where the soft tissue meets the bone.) Inhale gently and slowly through the left nostril until comfortably "filled up", then block the left nostril as well with your right ring and pinky fingers.
-Softly retain the breath on the inhale for a brief moment, then release your thumb and exhale through the right nostril slowly and fully. Then, inhale through the right nostril, another soft, very brief inhale retention with both nostrils closed, and finally, exhale fully out the left. This is one round. Practice nadi shodhana for 3-6 rounds.
-Finish nadi shodhana by exhaling out of the left nostril on the last round, then drop your hand, breathe through both nostrils a few times, and notice how you feel. Then go about the rest of your day, ideally with more clarity and vitality.
Practice Notes & Tips: The inhales and exhales should be even in length and quality. Never force, don't strain: simply follow the prana as it flows, from one nostril to the next, back and forth, up and down, as you relax and breathe fully. Ideally, the breath length should be a minimum of 5 seconds for the inhale and 5 seconds exhale. The length of the breath can be extended as you become more proficient, but again, never strain to do so.
Remember, don't abuse yourself or compete with yourself with any yoga practice. Be gentle, and care for the temple of your mind and body by taking it slow and easy, never forcing. Do 3-6 rounds with a 10 second breath (5/5) every day for a month and develop a good habit. Then perhaps try to increase as you feel ready to longer breaths for the next month, increasing until your breathing is slow, steady and easy. Our ancestors had much slower breaths than we modern domesticated humans do, and so perhaps one way for you to help our planet as an individual is to slow down and be mindful about your own breathing in this simple way! The slow and steady approach is better than blasting yourself with many, many rounds a few days, and then never doing it again. Dedicated, daily practice and repetition is key to learning and key to proficiency in ANYTHING. Give it a try.