You get this achievement in the
'One Night in the Woods' DLC/chapter of the game. It is unmissable as it is part of the story line. Solution to get this below. Credit goes to the Walkthroughking (
https://www.walkthroughking.com/text/apocalipsis.aspx).
-----
Take the shovel head from the side of the house. Move the shovel handle on the bench next to the house and use the shovel head on the handle.. Open the window and the door, then go inside.
Pick up the jar of glue and the pot plant, then also pick up the flint from the ground here. Open the chest and look at the papers inside to get a hint on what to do for Harry. Use the flint on the fireplace to start a fire. Go and collect the clippers from the hook on the wall near Harry. Place the pot plant on the bench to the left of the pot and it will grow. Take the leaves from it and put them into the pot. Head back outside.
Use the jar of glue on the broken shovel, then pick it up. Go left and use this on the pile of rocks, then take the arm. Return inside and put this into the pot on the stove. Take the pot and give it to Harry, then use the clippers to cut his hair.
----
Achievement will unlock right after cutting Harry's hair.