Can be achieved by placing different Cool style furniture pieces, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Cool indicator in the game.
More Cool furniture will unlock while researching different things at the decoration shrine and you will also be able to unlock some art pieces via friendship levels.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 Cool furniture of Cool: 2 stat, is not 4 stat, it's 2 stat.