Fairly straight forward achievement. The Flask is located in the Safe room on the 1st floor on the right side of the mansion. It will just be on the floor. The Lighter is located on the 2nd floor in the same room as the Dog Whistle(For Jill, if you play as Chris you already have the lighter). From the Safe room just head upstairs and through the door that is to the right of the stairs and down the hall. Once in the next hall head to the right of the door you came from to the room where you will find the Dog Whistle and the Lighter.
Now all that is left is to kill a zombie(or find a body just lying on the ground that isnt an enemy that is waiting to attack), stand over the corpse and use the flask. You will burn the body and earn this achievement.