firewns asked: “How can one meditate on reducing sanna and citta vipallasa about dukkha and asubha?”
One cannot reduce sanna and citta vipallasa DIRECTLY via meditation.
Those vipallasa are reduced when one cultivates panna by getting rid of the 10 types of miccha ditthi first, and then increasing one’s understanding on Tilakkhana (mainly the anicca nature).
– So, the meditation should be on Dhamma concepts.
– You may want to listen to the set of discourses on Tilakkhana:
“Three Marks of Existence – English Discourses“