This is from a handout during my fellowship training by Andrew Weil.
Ideally, sit with your back straight.
Place the tip of your tongue against the ridge of tissue just behind your upper front teeth, and keep it there through the entire exercise.
Exhale through your mouth around your tongue; try pursing your lips slightly if this seems awkward.
Exhale completely through your mouth, making a whoosh sound.
Close your mouth and inhale quietly through your nose to a mental count of 4.
Hold your breath for a count of 7.
Exhale completely through your mouth, making a whoosh sound to a count of 8.
This is one breath. Now inhale again and repeat the cycle three more times for a total of four breaths.
Do it at least twice a day. You cannot do it too frequently. Do not do more than four breaths at one time for the first month of practice. Later, if you wish, you can extend it to eight breaths. If you feel a little lightheaded when you first breathe this way, do not be concerned – it will pass.
Are the numbers important?
The absolute time you spend on each phase is not important; the ratio of 4:7:8 is important. If you have trouble holding your breath, speed the exercise up
but keep to the ratio of 4:7:8 for the three phases. With practice you can slow it all down and get used to inhaling and exhaling more and more deeply.
Why should I do it?
This exercise is a natural tranquilizer for the nervous system. Unlike tranquilizing drugs, which are often effective when you first take them but then lose their power over time, this exercise is subtle when you first try it but gains in power with repetition and practice. Use this new skill whenever anything upsetting happens– before you react. Use it whenever you are aware of internal tension. Use it to help you fall asleep.