You should unlock this through natural progression as the collectible plants in the game are required for crafting. Some quick tips:
- There's only 3 types of plants in the game (Bougainvillea, Yucca, and Cistanche) but any and all types that you collect will count towards the 15 you need for the achievement.
- If you wait until it gets dark out, collectible items (like the plants you need) are a little easier to spot because of their shimmer/glow.
- If you're grabbing every plant you see, you'll have this in no time!