Happy Wars

To unlock this achievement, you will need to add a buff to one of your items. In order to add a buff you an item, you must have an empty slot available on the item. You can check the item's stats to see if the item has an empty slot available. You also must have another item that has a white buff on it. It must be white, because the orange buffs are permanently attached to that specific item, and the purple buffs are negative and can only be removed. 

To add or remove buffs, go to Dress Up and Power Up, and then select Item Modification. Choose your item with an empty slot, and then choose the item with the white buff that you want added to your other item.

You will need some Happy Stars to perform the item modification, and then if you have enough, the achievement shall unlock.