To unlock this you need to place at least 50 decorations in your store. These can range from lights, to plants, to Halloween/Christmas decorations.
The easiest way to do this is to place lights inside of the store. As you expand your store make sure to put lights up on the ceiling and this will progress you towards this achievement. There is no way to check your progress towards this achievement other than the "Lights" tab it will show 'xxx/xxx' This is how many lights you have placed in your store.
This achievement will unlock after you close your store for the night after you have your 50th decoration.