How to do a long meditation of "death contemplation"?

because just thinking about it for a short time didnt have any effect on me - how should i do a long death contemplation meditation ? like half hour or an hour ? should i repeat a mantra over and over again ? And what to do when there are disturbances - should i focus on the words more strongly ? where my attention should be ? (doing death contemplation to gain a sense of urgency - chanda)