This is something you'll gain pieces of throughout the game in the forest kingdom.
While you're building structures, you'll occasionally get a "gift"/"piece" that a character will tell you to go get and put somewhere safe. Just do as they say and put them all somewhere you won't forget about them until the end of the game.
Once you finish the castle for the king, the final piece will be given to you along with the blueprints. Just pick up the blue prints and arrange the gift boxes as it says.
Be sure to put it somewhere you want it to be though, because;
****SPOILER******
They made you a rocking chair that you can sit in and relax! You earned it~