Things You'll Need
Instructions
Connect two Nintendo DS handheld consoles to the same wireless network. This can be done by clicking "Options" from the device's main menu and clicking "Wireless Networks." You can then select a wireless network you would like to join.
Load a saved game of "Pokemon Diamond" on one console and a saved game of "Pokemon Platinum" on the other console. On "Diamond" you will need to have already captured Shaymin. On "Platinum" you will need access to Floraroma Town, which is available near the end of the game.
Go to the top floor of any Pokecenter on both games. On the top floor there will be a variety of counters with nurses behind them.
Talk to the nurse behind the first counter on both consoles and click "Trade Pokemon." The trade screen will appear once it has located both Nintendo DS consoles on the wireless network, which typically only takes a few seconds.
Trade Shaymin to the saved of "Pokemon Platinum" by clicking on the Pokemon and pressing "Trade."
Travel to the center of Floraroma town. There will be a handful of people standing around, including a blond-haired girl.
Talk to the girl and she will give you a Garcedidia Flower.
Open your inventory and give the item to Shaymin. Once you give the Legendary Pokemon the item, it will automatically change to its sky form.
Return to any Pokecenter and trade the Pokemon back to your game of "Pokemon Diamond."