This achievement unlocks once you have successfully created an Iron Golem. This must be done in survival mode.
To complete this achievement you need 4 iron blocks (36 iron ingots) and a pumpkin.
Iron blocks are crafted using 9 iron ingots (fill all squares in table). Pumpkins are found in the wild on the surface, or can be grown if you find pumpkin seeds (located in chests in stronghold/abandoned mineshafts).
Now you have these blocks you can create the iron golem by placing them like this:
It should transform into this: