Can be achieved by placing different Cozy style furniture pieces, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Cozy indicator in the game.
More Cozy furniture will unlock while researching different things at the decoration shrine and you will also be able to unlock some Cozy Furniture 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 of the same Cozy furniture of Cozy: 2 stat, is not 4 stat, it's 2 stat.