Simply be aware of yourself from head to toe.
Draw around yourself, as it were, three circles of light.
Resolve that the mind shall not cross these three circles, nor will any intruding thoughts and impressions enter from outside.
Remaining aware of yourself from head to toe, in just a few exhalations, relax all your limbs.
Your inhalation is for the purpose of exhalation. So you inhale so that you may be able to exhale.
In each exhalation progressively relax your entire body.
Now resolve that for the next one minute no other thoughts will arise except the awareness of the movement of your stomach and navel area, with the gentle rhythm of your breathing. Maintain that resolve for one minute.
Now resolve that for one minute there will be no other thoughts except the awareness of the path of your breath from the navel to the nostrils, nostrils to the navel; only feel the breath flow on this path. Maintaining the resolve.
Now feel the flow and the touch of your breath in your active nostril; but first resolve that for one minute there will be no intruding thought, only the feel of the flow of the breath in your active nostril.
Now resolve that for one minute there will be no intruding thought except the feel of the breath in your passive nostril.
Resolve that for one minute there will be no other thought except the feel of the breath in your two nostrils.
Now with your breath, exhale thinking the word haaammm, inhaling the word soooo.
Resolve that for the next two minutes there will be no other thought except the feel of the breath in both nostrils, inhaling with soooo, exhaling with haaammm.
Maintain that resolve so that no break occurs between the exhalations and the inhalations.
Resolve for an additional two minutes that as you feel the breath and the word, you observe how the mind and the word are flowing together as a single stream, the mind stream itself becoming the word and the breath; maintaining the resolve for two minutes.
Now eliminate all dualities of the left and the right; come to the sushumna breath. Mentally feel the spot where the nose-bridge ends and the upper lip begins. Mentally feel the spot between the eyebrows.
Inhale as though you were inhaling from the spot in front of the nose-bridge, a subtle energy flowing to the spot between the eyebrows; exhaling the same way.
Maintain the same stream of the mind, word, and the energy flow.
This flow between the two spots is known as sushumna breath.
Resolve that for one minute there will be no other thought only the feel of the flow of the stream in the sushumna channel.
Now using the center between the eyebrows as the gateway, enter the chamber of your mind and resolve for a quarter minute, half a minute, there will be no exterior thought, only the utter silence; as though your mind becomes a lake of silence, absolutely still, without a ripple.
Resolve and maintain the enjoyment of such stillness and silence for a half a minute.
Now from that lake of silence, come again through the gateway between the eyebrows; and resolve for one minute to be only in the sushumna breath.
Maintain the same stream, without the intrusion of any other thought.
Now continue in silence. Only a single ripple arising in the silent stillness of the lake of the mind, and that is the mantra so-ham in the breath.
Every two-three minutes, five minutes, renew the resolve to permit no other exterior thought.
If you have difficulty with that, you may practice the entire process that you have just gone through, in repeated segments and steps described here.
Sit as long as you wish.