In diaphragmatic breathing you use your diaphragm to expand your lungs and press your belly outwards. This causes an inhale. You then can use your transverse abdominus to contract your belly and pull it inwards. This presses the abdominal organs upwards against the bottom of the lungs reducing their volume and causing an exhale.
The diaphragm is an umbrella or parachute shaped muscle that lies between the bottom of your lungs and heart and top of your liver, stomach, spleen and kidneys. When contracted it presses downwards causing the belly to expand if the abdominals are relaxed.
The transverse abdominus forms the innermost layer of abdominal muscle. Its fibers run horizontally around the waist between the bottom of the ribcage and the top of the pelvis. When contracted this muscle shrinks the waist and can be used to push the diaphragm upwards, reducing the volume of the lungs.
To practice diaphragmatic breathing, sit in a slumped position so that your abdominals are relaxed. The idea of slumping is to make it easier to use your diaphragm.
To begin diaphragmatic breathing, focus on smoothly pulling your belly inwards, towards your spine. Exhale as you do so. Smoothly relax your belly and inhale as you do.
Once you get used to this action, focus on feeling your belly as you pull it inwards and as you relax it. Try to feel the entire circumference of your entire waist contracting and relaxing and make this action as smooth as possible.
If you keep your ribcage still, when you contract your diaphragm it presses downwards. This causes your liver and stomach to press down. Meanwhile the rest of the abdominal organs compress and push the front of the belly forwards. Hence your belly expands when you inhale.
You can practice activating your diaphragm by trying to make yourself look pregnant. See if you can gradually make yourself look more and more pregnant while inhaling. Pull your belly inwards while exhaling.
If you put your awareness in the region of the bottom of your lungs you may notice a pressing down feeling as you inhale. That is your diaphragm activating. Once you can feel your diaphragm, see if you can press the entire surface of your diaphragm downwards while inhaling. Gradually relax the entire surface of your diaphragm while you exhale.
Now practice focusing on your diaphragm while you inhale and your transverse abdominus while you exhale. In each case, operate each muscle smoothly and completely. Practice moving your awareness from one to the other smoothly also. At the peak of your inhale move your awareness to contracting your transverse abdominus. At the peak of your exhale move your awareness to contracting your diaphragm.
Once you are comfortable with diaphragmatic breathing with your ribcage relaxed and sunken down, practice it with your ribcage slightly lifted. You may find this slightly more difficult becuase you add tension to your belly the more you lift your ribcage away from your pelvis.
Spend a minute or so practicing diaphragmatic breathing with your ribcage slightly lifted and then lift it higher yet. You'll have to work harder to push your belly out and also to pull it in. The advantage is that you'll be exercising your diaphragm and helping to stretch your abdominal muscles.
Work towards breathing diaphragmatically with your spine straight.
For the next exercise you can practice straightening your spine as you inhale. At the same time push your belly out. Then as you exhale let your ribcage sink down. You may get the feeling that as you inhale your diaphragm is actually helping to push your ribcage up. Then as you exhale you may get the feeling that the sinking action of your ribcage helps to press the diaphragm back up into your ribcage.
In the next exercise you can focus on pressing your diaphragm down but instead of allowing your belly to bulge out, engage your abs to pull your belly in slightly. You can then use your diaphragm and abdominals together to help cause your ribcage to lift. Relax both on the exhale so that the weight of your ribcage sinking down causes you to exhale.
One final variation that you can practice is similar to the previous breathing exercise however, this time you focus on keeping your lower belly pulled in. As you inhale, push your diaphragm down and allow your upper belly to expand. Your ribcage will still lift slightly as you inhale.
You can practice keeping your lower belly pulled in while inhaling and exhaling. You can also focus on relaxing your lower belly while exhaling and pulling it in at the beginning of each inhale.
Any method of breathing that uses the diaphragm can be called "diaphragmatic breathing." How do you know if you are using your diaphragm? If you can feel it contracting and relaxing.
So that you can learn to use your diaphragm whenever you like, practice feeling your diaphragm contracting when you press it downwards. To deepen your awareness you can focus on feeling your entire diaphragm when it is active. The diaphragm has three parts, you can imagine all three parts pressing downwards whenever you inhale. Another way of "feeling" or "sensing" your diaphragm is to visualize the bottom of your lungs and heart. Feel the bottom of each of these organs being pulled down when you inhale. Relax slowly while you exhale.
The more you focus on feeling your diaphragm the more likely you will be to feel calmer as a result.