Open your wardrobe, select the effects tab at the top (the one with a Habbo and a plus sign next to it, the last one). Select the Torch and then select Save Changes.
Place your Plumeria somewhere in the room. While you have the Watering Can be equipped, click the seeds and click on “use” until the flower has fully grown (12 clicks). Keep doing this until you have maxed out the achievement, which requires you to fully grow 80 flowers. When a flower has fully grown, it will turn back into a seed which will then allow you to re-grow the same flower instead of playing 80 different ones to use. However, sometimes the flower will turn into a weed furniture item, which will leave it unusable and will require you to place another Plumeria to complete the achievement.