Extracting from (with slight modification re retention) https://www.amazon.com/Wellness-Sense-practical-emotional-Ay...

>>> Respiratory cleansing purifies your entire body. It pacifies the three doshas and strengthens the seven dhatus. It flushes your lungs and blood with fresh and excess oxygen that results in definitive strengthening of your immune system and your overall wellbeing. Respiratory cleansing has a calm effect on your mind as well. It improves your memory, virility (or fertility) and strengthens your neurological system. According to the yogic texts, it allows the practitioner to live longer and healthier. It is called pranayama. Prana means vital life force and yama means to elongate it. The science of pranayama is a different subject matter and is not part of the mainstream Ayurveda.

Prerequisites
For all the breathing exercises, it is more rewarding to sit cross-legged. Sitting in this posture allows you better control on the flow of the vital energies in the body. In case, you can't sit cross-legged, it is okay to sit in a chair. Breathing exercises should not be done while lying down. In all the exercises, your back and neck should be in one straight line. Your posture needs to be firm and straight but not tense. It is best to do these exercises on empty stomach in the morning. If you are doing it during the day or at night, make sure there is a minimum of two-hour gap between your meal and the exercise. And, that's assuming you had a light vegetarian meal because a light vegetarian meal completely digests in two hours.
If you had a heavier meal, you may want to increase the gap to three hours or more. Start all breathing exercises with an exhalation first. This is a subtle but extremely significant point. Exhalation allows you to expel toxic air in your system. If you start with an inhalation, you simply pressurize the foul air to circulate through your system. A general rule of thumb is to be followed for all yogic exercises: if they make you uncomfortable, stop right away and seek expert guidance.

Simple Deep Breathing
Simple deep breathing is a hassle-free, potent and easy exercise. Assuming you have followed the prerequisites, just sit comfortably, rest your hands in your lap if you are sitting cross legged or on your knees if you are sitting in a chair. Start with exhale as per prerequisit. Inhale deeply and gently with both nostrils. Fill your belly and lungs with fresh air. Do not hold it and instead start exhaling gently. Pull your stomach in as you exhale and gently push your stomach out as you inhale.
Simple deep breathing can be done for fifteen minutes in the morning. Do it for five minutes and take a break of two minutes and then do it for another five minutes.

Alternate Breathing
Alternate breathing is a type of pranayama that is excellent for neurological and respiratory cleansing and detoxification. It forms part of the nervous-system-purification (nadi-shodhana) regime. It is called anuloma-viloma in yogic texts.
To do alternate breathing, follow the prerequisites. As always, start with complete exhalation with both nostrils. Put the thumb of your right hand on your right nostril to close it. Now breathe deeply, steadily and gently through your left. DO not hold the breath. Put the middle finger of your right hand on the left nostril and lift your thumb to open the right nostril. Exhale completely, steadily and gently. Ideally, your exhalation should be so soft that you should not even hear yourself breathing out. Yogic scriptures state the standard one-four-two rule for pranayama. It means if it takes you one second to breath in, for example, you should hold the breath for four seconds (four times the length of inhalation) and exhale over two seconds (double the length of inhalation). However, as I stated earlier, retention of breath should be done only if you have been guided by an expert and if you are observing all the rules.
At one stretch you can do twenty repetitions. One complete repetition is inhale from the left, hold, exhale from the right, inhale from the right, hold, and then exhale from the left. This is one repetition. If you have the time, you are free to do it twice or even thrice a day. There is no better purifier of your entire nervous system than pranayama. It is nothing short of a miracle exercise handed down to us by the ancient yogis.