At the start of each level, you will head to the Upgrade Shop. Here you are able to spend your hard-earned cash to purchase and upgrade various items and assistants in the spa. To unlock this achievement, you must purchase and upgrade everything fully. This includes all of the chairs, all of the candles, the magazine table, the tea machine, all of the assistants, and Nell's product shelf. The items in the spa will gradually become available for purchase as you progress through the story. You will not have enough cash to buy them all until after Level 20.
NOTE: Do not purchase the cashier assistant until you earn Perfect Score on all 25 levels. Doing so will make the game much more frustrating and difficult to earn Perfect Scores, as the cashier assistant will automatically cash out customers, even if they do not have 5 stars.