As given by Swami Satyanananda Saraswati.
Just breathe. Be aware of your natural and spontaneous breath. Be aware of your ingoing breath and be aware of your outgoing breath. No effort. Your breathing pattern can vary, sometime your breaths are short, sometimes they are long. Now when you inhale add So, and when you exhale add Ham. You are affirming that you are saying So with your ingoing breath and Ham with your outgoing breath. You are not making any loud sounds, it is normal breathing. You can do this anywhere and anytime. Practice for 2 to 3 minutes, or longer. You will feel calmer and centred. It will enable you in your daily meditation.
This little sadhana was given in1968. It is wonderful, try it, share it with kids, your loved ones and on people who are sick.
Aim Hrim Klim