What is the most important ritual in a Kartik Poornima pooja?

The most important ritual is the sacred bath (Ganga Snan) taken before sunrise at Brahma Muhurta, followed by Deep Daan — the offering of ghee lamps. These two acts together are said to grant inexhaustible merit (akshaya punya) and destroy sins accumulated over many lifetimes. The Sankalpa recited before these acts anchors the merit to the devotee personally.