If you do concentration during meditation, Pranayama comes by itself.
The breath becomes slower and slower.
For example, when reading a sensational storybook or when solving a mathematical problem, your mind is really very much absorbed in the subject matter. If you closely watch your breath on these occasions, you will find that the breath has become very, very slow. Pranayama comes by itself.
In those Yogic students who practice Sirshasana, Pranayama comes by itself. It is obvious from these examples that when the mind is deeply concentrated on any subject, the respiration slows down or stops. Pranayama is being done automatically.
Mind and Prana are intimately connected. If you turn your attention to watch the breath on those occasions, it will regain its normal state. Pranayama comes by itself to those who are deeply absorbed in doing Japa, Dhyana and BrahmaVichara (enquiry of Atman).
Just as there is connection between wind and fire (light), so also there is connection between Prana and mind. Wind fans the fire. Prana also fans the mind. If there is no wind the fire or light becomes steady. Hatha Yogis approach Brahman by controlling Prana. Raja Yogis approach Brahman by controlling mind.
In Pranayama during Meditation you need not close the nostrils. Simply close the eyes if you practice it in a sitting posture. Forget the body and concentrate on the breath. If you practice this during a walking meditation, just feel the movement of the air that is subtly inhaled and exhaled. You will then practice this Pranayama daily, unconsciously.