Instructions
Travel to the Desert world, to the area marked zone 9 on that map. You can see the map on your heads-up display or on the loading screen when you travel between zones.
Defeat any monsters that may attack you so you can freely explore the map. You do not need to attack all of the monsters in the zone, only ones that may impede your digging.
Locate a copse of cactus plants, and use the "Dig" function near them. A context-sensitive command will appear over your character to let you know that you can dig. Press the corresponding button on your Wii Remote or Classic Controller Pro. The button may differ based on the controller, but is usually "A."
Continue to dig around the cactus plants until you locate some cactus flowers. You may need to dig a few times before a cactus flower appears at random. A message will appear on the screen informing you that you've collected an item.