you can't, you have to have a gameboy advance Pokemon game (emerald, ruby, sapphire, fire red, leaf green) and migrate a ditto. ditto is found in emerald in the digger's house cave(not sure if it's in ruby or sapphire). migrate is an ability received after beating elite four.
Breed MANAPHY and DITTO together
no,ditto is not in route 218 in Pokemon platinum it is in the trophy garden
get a manaphy egg in shadows of almia and transfer it to platinum then hatch it and put it in the daycare with a ditto or get a phione egg from a from
ditto
Breed Ditto and Manaphy. Ditto + Manaphy = Phonie Ditto + Phonie = Phonie
try breeding it with a ditto.
breed manaphy with ditto
the global trade station
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The Trophy Garden is the only place to get Ditto in Pokemon Platinum, without trading or cheating of course.
you can catch a Ditto if you talk to Backlot every day for a long time. He will say that he saw a ditto there. Then, you can catch a ditto in the trophy garden.
If you want to see one try route 218 outside canalave with the pokeradar.