Many pranayama breathing exercises require the practitioner to restrict the air entering and leaving the lungs. To do this the more dominant arm is bent and the biceps are placed onto the front, side part of the ribs so that the arm can relax and does not fatigue during practice.
The thumb and a chosen finger are used to control the incoming and outgoing air through the nostrils. Choose the finger that is most easy to use. Traditionally the ring finger was used and there are many reasons for this choice but if desired other fingers can be used exclusively or in alternation.
Find a comfortable seated position. Gently exhale all of the air from the lungs.
Bend one arm and clasp the nose with the thumb and ring finger.
Lay the bent arm along the side of the chest.
Begin to restrict the air going in through both nostrils. Feel the air going in through the nostrils and restrict them accordingly so that an even intake of air passes through both.