Instructions
Acquire Arceus. Trade for him through another "Pokemon" game, or attend a Nintendo Event (see Resources) to obtain the Azure Flute for the games "Pokemon Diamond," "Pearl" or "Platinum." Use the Azure Flute to open the Hall of Origin on the top of Mt. Coronet. Catch Arceus inside. Arceus is not obtainable directly in "HeartGold," so you must obtain him through these two methods.
Travel to the Ruins of Alph. Head south from Violet City, towards Route 32. Look for scientists outside the ruins to identify the entrance. Make Arceus your companion Pokemon. Talk to the lead archaeologist, who asks you to enter the ruins with your Arceus Pokemon. The screen fades, and you and Arceus teleport to the Sinjoh Ruins.
Find the cabin south of the ruins. Deposit all of your Pokemon in the computer inside the cabin. Make Arceus the sole Pokemon in your party. Speak to Cynthia, who leads you into the ruins to show you the Mystri Stage.
Pick your Pokemon. You can pick Dialga, Palkia or Girantina, all of which will start at level one. Select Girantina.
Trade for Giratina instead of acquiring Arceus and going to the ruins. Find a friend or another copy of the game that has a Giratina on it.