Step #1: Finding a city you want to mass murder.
For me, I have chosen Faiyum.
Step #2: Get a bad guy.
It dosent matter what bad guy, but it cant be a civilian.
Step #3: Finding a good spot for the murder.
You'll want to find a place where many people pass through, a 'choke' point if you will.
Step #4: Start the killing spree.
Place your dead bad guy down. Use your flesh decayin poison (Seer treeline).
Step #5: Enjoy!