In the beginning of Chapter 3
Go back to the zombie's home.
Introduce Ethel and Esther to each other.
Talk to Esther and Ethel.
Go back to the scrap yard.
Put the long wooden board onto the sewage pipe in the middle of the area.
Talk to N8.
You fancy a quick seesaw?
Put Timmy onto the seesaw.
Talk to N8.
I could do with you as extra weight, N8…
Achievement will unlock after this sequence