Start this exercise any time you experience stress or shallow breathing.
When you notice your chest tightening up, shallow breathing, anxiety, or your body being in fight or flight mode because of stress say, "STOP!" (In your head if people are around, out loud if no one is around. Saying things out loud is 10x more powerful than just thinking it)
Take deep breaths in through your nostrils and breathe out through your mouth.
Do it 10 times.
Inhale and exhale so your lower lungs and upper longs expand and empty.
Now add mindfulness to it by:
1. Inhale slowly through your nostrils and imagine your lungs filling up with air from the bottom of your lungs to your upper chest. While you do this say or think, "This is my in breath."
2. Exhale slowly through your mouth and feel your abdomen shrink and your lungs empty out the air. While you do this say or think, "This is my out breath."
3. Repeat this 10 times.
4. Notice the sensations in your body
Once you try it with mindfulness, let's take it to the next level.
While you do step 1 picture yourself inhaling stress.
While you do step 2 picture yourself exhaling stress in the form of black smoke coming out from under your feet, exiting your body, and into the ground.
Thich Nhat Hanh's video on Mindful breathing here.