To develop a larger breath, encourage the diaphragm to become more active. Imagine the breath moving deeper into your belly on the inhale. Allow your belly, lower ribs, middle ribs, and upper chest to expand on the in-breath and contract with the out-breath. Work to allow all areas to expand and contract all the way around your body and not just in the front. Try to breathe through your nose at all times if possible.
The goal is to have a deep, slow, and smooth breath. You want to breathe in a breath that fills your lungs with ease, starting low in your belly and working your way up. You want to breathe out to expel the maximum stale air possible while maintaining a gentle, easy, and comfortable breath. The better we become at exhaling, the more toxins and stale air will be eliminated. This helps to bring in more oxygen-rich air on the next inhale. Just remember to be gentle and not force the breath.
Do the exercises for three to ten full breaths each time. A full breath is one inhale and one exhale. These exercises can be done anytime while sitting, lying down, or walking. The trick is to breathe gently. The breath cannot be forced, but you can influence the breath to some degree. It needs to be easy and soft. This is a balancing act. If you push too hard, it will cause stress. If it is too soft, you will not grow a full breath. If possible, breathe through your nose. Always check with your doctor before beginning any new exercises.
If thoughts take you away from your focus on the breath, just let go and move your attention back to the breath. If whatever pulled you away is strong, let your attention stay on it until it lessens, and then return your focus back to your breathing.