Things You'll Need
Instructions
Turn on "Pokemon HeartGold" and begin to progress through the land of Johto. Raise up your party to be strong enough to catch a Growlithe upon encounter. If a friend is helping you get a Growlithe, they may already have a strong party in place.
Encounter and catch a Growlithe. These "puppy Pokemon" can be found on routes 7, 8, 36 and 37. Walk around the area and battle the wild Pokemon until a Growlithe appears. Weaken the Growlithe, then catch it with a Pokeball.
Turn on "SoulSilver" on the other DS. Have the characters in both games head to the Pokemon Center and proceed to the second floor. Have the player in "HeartGold" stop and put Growlithe into his party if he is not already there.
Talk to the woman in front of the center door to enter the Union. This is where you can complete trades. Have both players do this at the same time.
Complete the trade by sending Growlithe to SoulSilver in exchange for another Pokemon. Growlithe will now be part of your party and you can use him in battle.