1) Start your Pokemon Diamond or Pokemon Pearl game.
2) Go to a Pokemart and save the game.
3) Restart your DS and go to the title menu for your game.
4) Using a second DS start up Pokemon Ranger
5) Enter Ranger Net and check on your egg.
6) Start sending the invitation.
7) Diamond and Pearl will pick up the signal and begin to walk you through the process.
8.) After a few taps on the touch pad the egg will be transferred to your Diamond or Pearl game.
9) Go back into the Pokemart and a man in green will be there to give you the egg!
It is that simple!
What's that you say? You don't have Pokemon Ranger? Well there is a way to get a Manaphy that is not quite as good as the real thing. See, Manaphy is the first legendary Pokemon that can breed.
If you know someone who has a Manaphy they can breed it with a Ditto to hatch a Phione (#489). This Pokemon is a more "generic" Manaphy and has base stats 20 points lower than the real thing. It also lacks two of Manaphy's signature moves; Heart Swap & Tail Glow. Everything else is the same.