This is not a missable achievement, wich means you can get it after finishing the story.
The first time you can get it is after you beat the forest robot and get the water hat.
Without further ado, here is what you gotta do:
First of all you need an item called "The Golden Tooth from the Silver Frog"
or something like that, wich can be found at the end of what I'll call a "dungeon", located at the forest, near where you find the insect there. I'll leave a screenshot here.
You must drop the "secret" entrance on the right of where I stand in this pic and go all way to the right to find a door and enter the "dungeon". I wont be saying what you gotta do there, since its pretty linear and its not the objective of this guide.
After acquiring the tooth, you must go a house in the valley village (pic below), where there is an indoor garden and water the flower on the middle. This will cause it to grow outside the house, allowing you to go up in the clouds (like in 'Jack and the Beanstalk').
Up there just enter the airship and talk to the wizard inside, he will grant you some fake and ugly arms that do, well, nothing and the achivement will pop.