Not to be confused with the "Top Magatama Idol" achivement which has a similar description.
For this achievement, you will be actually modifying the magatama themselves. These can be collected via any means. Once you reach Kyoto and complete Kaguya's quest she will have a new option to ornament the magatamas. These will enhance the magatamas with additional effects beyond what they already have. As Kaguya's friendship level increases she will have better quality ornamentations (but they require harder to get materials).
NOTE: In addition to completing the aforementioned quest, Kaguya must be at least friendship level 5 to access the ornamentation option.
For more information about Magatamas and how they work check the Onigiri Wiki page: http://onigiri.wikia.com/wiki/Magatama