Nadi means little river, and, as we have seen earlier in our journey, it refers to the channels through which prana flows. Nadis are equivalent to the meridians. Shodhana means purification. Thus, nadi shodhana is a cleansing of the energy passages. Other names for this practice are alternate nostril breathing or anuloma viloma.  The practice not only cleanses the nadis, it balances the energies on both sides of the body.
The hand position is unique for nadi shodhana. The right hand is used, and the middle two fingers are either folded down to the palm or extended so they can rest on the spot between the eyebrows. The right thumb is used to press in on the right side of the nose, closing that nostril. The little finger and ring fingers are kept together, and are used to close the left nostril. Since the right arm will be kept lifted up during the practice, it may get heavy – students can use the left hand to support the right arm.
We begin with the left side: exhale, and then use the right thumb to press against the right nostril, closing it. Now inhale for a count of four through the left nostril, then release the right side while closing the left side, and exhale for a count of four. Complete the cycle by inhaling on the right side, close it and open the left side then exhale. Continue with a four count for eight to twelve cycles. When finished, sit quietly.
Adding kumbhaka and lengthening the exhalation
A more advanced version of nadi shodhana keeps the same inhalation timing, but extends the exhalation for eight counts. When this is mastered, you may wish to add retentions. Between the inhalation and the exhalation, pinch both sides of the nose closed, and retain the breath for four counts. This is antar kumbhaka: retention with lungs full. As you gain experience, you may add bahir kumbhaka at the end of the exhalation, also for four counts.
More advanced, extended nadi shodhana practices are available after a few months of practice with these simple variations, if you experience no difficulties or side effects.