Instructions
Go to the PAL Park in "Pokemon Diamond," which is marked on your in-game map. Have your friend go to the PAL Park on his own copy of either "Pokemon Diamond" or "Pokemon SoulSilver." Enter the trading room.
Walk up to your friend's avatar and press "A." Select "Trade" to initiate the transaction.
Select the Diamond Pokemon you want to transfer. Confirm that you will trade it to your friend.
Turn off your game and put in your copy of SoulSilver. Have your friend stay in the room.
Enter the same trading room and initiate a trade. Have your friend send the Diamond Pokemon you gave him to your SoulSilver game.