Living in heart coherence simply means being present, breathing in and out through the space of your heart and intentionally feeling a heart based emotion. Gratitude is a perfect emotion, love is also wonderful to feel.
The longer you can stay centered in your heart by bringing your attention to it and breathing in gratitude, the better you will feel. When you feel good, you make good decisions that will lead to more good feelings. Heart coherence will help you to spiral up more and more.