You need to read Geppetto's letter placed at his desk in Hotel Krat after 'Real Boy' ending for this achievement.
Timestamp: 09:06:45 (Part 2 video)
To get the 'Real Boy' Ending you need to give your heart to Geppetto when he asks for it. (there is no need for any humanity for this ending.)
Timestamp: 09:02:40 (Part 2 video)
After the cutscene cancel the option to go to New Game plus and you will be back at Hotel Krat and all the humans are now replaced with Puppets. Make your way to Geppetto's desk and letter.
Timestamp: 09:05:05 (Part 2 video)
100% Complete Walkthrough for all achievement/collectibles and Full Game: