Pranayama is the mastery and control of the breath. The breath influences the mind, breathe is energy we use to live. Breathing in special patterns changes the energy-flow in the body and mind. Alternate nostril breathing, or anuloma veloma, or nadi shodana, is an ancient technique, which is helping us to calm the mind, open the breath-ways, and to balance emotions.
The basic ratio is 1 part inhaling, 4 parts holding the breath, 2 parts exhaling.
An intermediate practice is with 4 seconds inhaling (16 seconds holding the breath 8 seconds exhaling).
Advanced AV is 5-10-20, 6-24-12, and so on.
It is better to start slowly and when you get comfortable with the rhythm you can go slower and slower.
The most important thing is to learn the exhale. Do not exhale too fast. Stretch it over the whole time. If you exhale to fast, you will be out of breath very soon.
If you get out of breath, breath with the mouth in between 1 time, then keep following the rhythm.
A very beginner can start without holding the breath. Inhale 4, exhale 8. Then start with 4 seconds retaining the breath, build it slowly up to 8 and 12, then 16.
Do it with eyes closed. It is a meditation by itself. After the practice leave your eyes closed, whether start your meditation, or simply feel the effects of this calming pranayama.