What are the most important Kartik Poornima rituals to perform?

The three most important Kartik Poornima rituals are: (1) the pre-dawn holy bath (Ganga Snan) taken at Brahma Muhurta, (2) Deep Daan — the offering of ghee lamps to Lord Vishnu and Mother Ganga at sunset, and (3) the Sankalpa-based Vishnu Puja with the Damodara Ashtakam. Night Vigil (Jagarana) and Charitable Giving (Daan) are also highly meritorious. Together, these rituals constitute a complete Kartik Poornima observance.