To practice this way, as soon as you are aware of a lack of integrity or some slouching in the body, begin with the foundation, the feet. Push through the feet firmly but gently, and feel the counter-current that helps you to engage navel toward spine comfortably and open the heart up + out through the shoulders. Feet, navel, chest... Foundation first. That is mainly it.
Once you create that kind of structural integrity, notice the difference and notice the breath. Then the rest is just being alive and enjoying it... Mindfulness indeed.