Exercise 1: Single Nostril Breathing

Sit in any one of the meditative poses (or sit on a chair in a pose called the "Egyptian Seat") keeping the spine, neck and head in a straight line. Close the right nostril with your thumb. Inhale, slowly, through the left nostril, counting mentally five times. Exhale through the same nostril, while counting ten times. That makes one round. Exhalation time is always twice the inhalation time. The proportion is 1:2. Repeat this exercise fifteen to twenty rounds through the left nostril, keeping the proportion five seconds inhalation and ten seconds exhalation.

Now, close the left nostril with your right ring finger and little finger, and inhale through the right nostril. Count five times for inhalation. Exhale through the right nostril while counting ten times. Repeat for fifteen to twenty rounds.

Do not make any sound during inhalation. Apply the basic rule of low, mid and high breathing, during inhalation. In exhalation, try to expel as much as possible of the foul air from the lungs.

You should practice Exercise 1 for fifteen days and, then, slowly, increase the proportion to six seconds inhalation and twelve seconds exhalation. Do not attempt the higher proportion until you are able to do the lower proportion very easily. This is the main rule in every breathing exercise. Always keep within your capacity and never overdo.

In Exercise 1, there is no retention. The purpose of inhaling and exhaling through one nostril is to correct wrong breathing habits. 
Unless one is able to do low, mid, and high breathing perfectly and automatically, he should not attempt advanced pranayama. Practice Exercise 1 for at least a month, even though you may feel the urge to extend the time. If you have a strong foundation, you can build a strong building.