For this achievement you need to collect 42 costumes in the game.
Costumes are hidden in pink coloured shopping bags. Each of the regular levels in worlds 1 to 4 has one such bag.
Since videos on the collectibles are very rare at the moment I ll use the once that I have also used in my solution for the 100% achievement.
I will edit this solution as soon as something new comes up or when I have done some videos myself. EDIT: There are a total of 11 costumes from the shop in Horizon Ruin. As MysticWeirdo points out in the comments: "There are 10 in the shop, 5 available after the 3rd boss, and 5 after the 4th boss.
You get one automatically for beating the final level"
Down below you find videos that cover all collectibles in all worlds , but the volcano, and in the hub / crono castle level. (video credit: Macktion) World 1World 2World 3World 4Hub level & crono castle
Hope this helps you guys.
Any improvements? Please let me know in the comments below!