After mixing all 3 ingredients together in the cauldron in the basement (Witch's hair, Puppy dog tail and twig doll) after a few seconds it should spawn in a twig doll in the cauldron. Click on the doll to pick it up and to the left of the cauldron there is a wall with a lot of spikes on. Click on one of the spikes to impale the doll. Doing this will unlock the achievement.