Instructions
Load a copy of "Pokemon Diamond" into one Nintendo DS and a copy of "Pokemon Pearl" into a second DS.
Walk the main character in both games to a Pokecenter and go downstairs. You will find three booths with people standing in them. Speak with the woman in the middle booth to go into the Trading Center. Do this for both games.
Follow the onscreen instructions to initiate a trade between the two games. When prompted, offer a Slowpoke or Slowbro from the copy of "Pokemon Pearl" for trade. Offer any Pokemon from the "Pokemon Diamond" game and confirm your choices on both systems to complete the trade.
Evolve your newly acquired Slowpoke, if that's what you traded, into Slowbro by getting it up to level 37. Do this by taking it into battles to give it experience, or by feeding it Rare Candy. Each Rare Candy raises a Pokemon's level by one.