To get the Poop Head, you must first find the Teddy Head (allows for a third jump) in Robot World:
Then start a new game with the Teddy Head and play through the Cave World. After beating the boss and reaching the area with Sally, approach her and use a triple jump (hold
to jump higher) and you will be able to climb on top of her. Drop off to the right and enter the world snake coming out of her to then appear in a small area with several ghost enemies and the Poop Head.
Pick it up (stand over it and press
to swap heads) and the achievement will unlock.