Dandelions can appear at random in your forest or if you use a butterfly ability to spawn one. Interact with these by tapping and selecting a butterfly and then dragging to fly it through the dandelion. This will release all the floating seeds and you then need to fly your butterfly around and catch as many seeds as possible before they disappear! Seeds will begin to flash before they disappear to let you know time is almost up. Catching seeds earns you honeydew and counts toward goals in your Flutterpedia.