That's good question. If you are know-nothing yoga practitioner, you can start the pranayama practice with abdominal deep breathing first. In this breathing, sit comfortably and silently either on the ground or chair. Then put one of your hands on the abdomen to guide it to come out when you inhale and go inside towards the spine, when you exhale. This makes one round. Breathing should be normal and slow. Repeat this practice for 5 rounds on day 1. On day 7, increase the rounds to 10. This time, there is no need to put your hand on the abdomen as by now. Keep your hands in chin mudra or anjali mudra. On day 14, the breathing should be deep and slow and increase the rounds to 15-20. After you master abdominal breathing, you are now ready for Naada Anusandhana in which there are three stages of breathings - abdominal, chest and then shoulder breathing, followed by all three combined in one breathing. So, this should prepare you for the pranayama which includes inhalation, retention and exhalation in a particular ratio.
While writing the above suggestion, I presume that you do not have any kind of respiratory or blood pressure disorder, else you should first consult your physician before practising the above exercise.
