This achievement is unlocked by crafting fifteen different potions. Crafting the same potion fifteen times will not work. As you play through the game you will automatically acquire potion recipes. Alternatively you can purchase them from apothecaries and occasionally find them by looting corpses. The first alchemy bench you are likely to encounter is located behind the apothecary in Rattay.
You can monitor your progress by navigating to: Player > Statistics > Craft > Unique potions brewed.