How to reduce stress through breathing exercise

BREATH AND STRESS
As you inhale, place your tongue on the roof of your mouth right behind your teeth, then breathe through your nose and slowly count down from five; on the exhalation, let the air escape through your mouth and count back up to eight. Then repeat. This helps you to really empty your lungs and relax into each breath.
This breathing exercise variation has been practiced for thousands of years as a form of meditative breathing. As you inhale, place your finger over your right nostril and only breathe through your left. On the exhalation, switch nostrils and only breathe through your right. You can breathe at whatever pace is comfortable for you, either a 5-8 ratio, a 4-7-8 ratio or whatever pace feels most relaxing for you (see "counted breathing," above). Repeat this exercise for up to five minutes.
Get into a comfortable position, close your eyes, and start to notice your breath. Before you begin to alter it, pay attention to the pace and depth. Are you taking deep breaths or shallow ones? Are you breathing quickly or slowly? (Becoming aware of your breathing can help you to become more mindful of your body's response to stress, and can help you to notice when you need to deliberately relax your breathing.)
Sometimes all you need to release stress from your shoulders, back, or the rest of your body is a few big, cleansing breaths.3 Breathe in deeply through your nose, and take in as much air as you comfortably can. Then release it, and really focus on emptying your lungs. (Many people hold air in their lungs after an exhalation, so emptying your lungs on a deep exhale can help you to get more fresh oxygen into them.) Repeat this breathing exercise for a few breaths and release the tension in your back, your shoulders, and anywhere else it tends to reside.
Experiment with whatever ratio feels comfortable to you, and see if it helps you to feel relaxed. The act of counting as you breathe still helps you to maintain a steady pace and keep your mind on your breath and the present moment, so it is still more effective than simply breathing regularly and unconsciously.

You may want to try different relaxation techniques to see which one works best for you. And if your favorite approach fails to engage you, or you want some variety, you'll have alternatives. You may also find the following tips helpful:

Choose a special place where you can sit (or lie down) comfortably and quietly.
Don't try too hard. That may just cause you to tense up.
Don't be too passive, either. The key to eliciting the relaxation response lies in shifting your focus from stressors to deeper, calmer rhythms — and having a focal point is essential.
Try to practice once or twice a day, always at the same time, in order to enhance the sense of ritual and establish a habit.
Try to practice at least 10–20 minutes each day.

Breath focus helps you concentrate on slow, deep breathing and aids you in disengaging from distracting thoughts and sensations. It's especially helpful if you tend to hold in your stomach.

First steps. Find a quiet, comfortable place to sit or lie down. First, take a normal breath. Then try a deep breath: Breathe in slowly through your nose, allowing your chest and lower belly to rise as you fill your lungs. Let your abdomen expand fully. Now breathe out slowly through your mouth (or your nose, if that feels more natural).

Breath focus in practice. Once you've taken the steps above, you can move on to regular practice of controlled breathing. As you sit comfortably with your eyes closed, blend deep breathing with helpful imagery and perhaps a focus word or phrase that helps you relax.