1. Yogic breathing:

Before we proceed to actual breathing practice, it is important to understand and practice diaphragm breathing, also known as yogic breathing. In many of us, when we inhale, our abdomen contracts and when we exhale our abdomen expands. If this is the trend of breathing, it should be modified in such a way that when we inhale, our abdomen should expand and when we exhale, our abdomen should be contracted to the extent possible

2. Nāḍi Śodhana Prāṇāyāma:

(Based on Haṭhayoga Pradīpaka and Yoga CūḍāmaṇiUpaniṣad)
Haṭhayoga Pradīpaka has allotted an entire chapter (chapter 2 consisting of 78 verses) for prāṇāyāma. This chapter begins by saying, “After mastering āsana-s, attaining self-control and diet control, one should practice prāṇāyāma. When the breath is not steady, mind is also not steady. When nāḍi-s are impure, prāṇa does not enter suṣumna. Nāḍi-s’ impurities can be removed by inhaling through left nostril and after holding as long as possible, exhale through the right nostril. Again inhale slowly through the right nostril (through which exhalation was made) and again holding for a specified time, should exhale through the left nostril. This should be practiced for three months and all the nāḍi-s will be purified.”  Yoga Cūḍāmaṇi Upaniṣad also conveys the same meaning, except that it says that complete purity of nāḍi-s can be attained in two months of practice.
There are twelve stages in nāḍi śodhana prāṇāyāma. Stages depend upon the duration of inhalation (pūraka), holding of breath within (kumbhaka) and exhalation (rechaka). For practicing the first three stages, no particular sitting postures have been suggested. However, it would be ideal to sit in padmāsana (lotus posture) or (ardhapadmāsana) half lotus posture. These postures will be discussed in later parts of this series.
To begin with, one can take any comfortable posture to sit. While sitting, there should be no tension in any parts of the body. Little finger and the ring fingers should be used to close the left nostril. Middle and index fingers should either be folded or extended outwards.  These two fingers should not be placed at ājñā chakra, as this could cause varying degrees of pressure on both the nostrils. Thumb should be used to close the right nostril. While closing the nostrils, very moderate pressure is to be applied on the nostrils. Similarly, instead of closing the tip of the nostrils, an inch above the tip of nose can be used to block the breath. However, one should not hold the breath beyond their holding capacity. Holding capacity will increase over a period of time. Always inhalation should be faster and exhalation should be longer.
There are twelve stages in nāḍi śodhana prāṇāyāma. All the twelve stages can be mastered in a matter of two months. It is easier to practice the first six stages and it is going to be difficult to practice the next six stages, as the holding period and exhalation period is more here. Holding period mentioned here are only indicatory and one should not hold breath beyond one’s capacity. Holding breath beyond one’s enduring capacity will even cause death.
In two months, we have 60 days and the chart below will explain how all the twelve stages can be practiced in 60 days. This chart is only indicatory in nature. Numerals in inhalation, retention and exhalation are only in counts and not in seconds. Approximately 1 to16 can be counted in 8 seconds. Let us take the example of retention period in XII below. This means that breath should be held here for 32 seconds or less. But this will be too strenuous. Therefore, it is advisable to stop at VII below and further practice should be based on one’s holding capacity.
Stage
inhalation
retention
exhalation
ratio
duration
I
slow
nil
slow
NA
3 days
II
slow
nil
slow
1:0:1
3 days
III
5
2
5
1:0.5:1
3 days
IV
5
5
5
1:1:1
3 days
V
2
8
4
1:4: 2
3 days
VI
4
16
8
1:4:2
3 days
VII
6
24
12
1:4:2
4 days
VIII
8
32
16
1:4:2
5 days
IX
10
40
20
1:4:2
6 days
X
12
48
24
1:4:2
8 days
XI
14
56
28
1:4:2
9 days
XII
16
64
32
1:4:2
10 days
TOTAL
 
 
 
 
60 days
 
This schedule is for a period of sixty days. One can make minor adjustments in the number of days. In particular, Stages VII to XII should be practiced very carefully. One should not hold breath beyond one’s capacity. Once stage XII is reached, one should continue to practice this forever. Further, unless perfection is attained in a particular stage, next stage should not be practiced. It is also not mandatory that one should follow this 60 day schedule and can comfortably be extended to suit one’s convenience. From stage VII, one can allow sufficient gap between two rounds. Numbers in inhalation, retention and exhalation are counts. One can count mentally. The basic principle is that inhalation, retention and exhalation should be in the ratio of 1:4:2. Under any circumstances, retention should not exceed 4 times of inhalation, As far as exhalation is concerned, counts be increased if possible. As a principle, inhalation should be faster and exhalation should be slower.
It is important that prāṇāyāma, bandha-s and mudra-s should be practiced only in empty stomach and should not be practiced when hot weather prevails. It is advisable that these practices should be done only during early morning or late night, when stomach is empty. In general, there should at least be three hours gap between food and practice. Any deviation from the prescribed norms will cause general health impairment.
Further Readings:
Importance of Breathing - Svara Yoga
Breathing Exercise
Stretching Exercise