Sit in any comfortable asana. Any of the traditional asanas, such as padmasana, siddhasana, etc. are best. Your spine should be held upright throughout the practice.
Control of breath through the nostrils
During the practice, the breath flow has to be directed firstly through the left nostril, then through the right nostril and then through both nostrils together. The method of control is to use nasagra mudra.
The right hand is normally used, but if you wish you can use your left hand. The second and third fingers should rest firmly on the forehead. The little finger is not utilized. The flow of breath through each nostril is controlled by the thumb and ring finger. If you use your right hand, then flow of breath through the left nostril is induced by pressing the thumb against the right nostril. Conversely, breath flow through the right nostril is induced by pressing the ring finger against the left nostril wall. If you use your left hand for control then the same will apply, but with the roles of thumb and right finger reversed.