How do you catch a Shadow Mew in Diamond?

You can't catch Shadow Mew in Pokémon Diamond.

* Shadow Pokémon are a feature of Pokémon Colosseum and XD Gale of Darkness which are GameCube games.

* Mew is not a part of the Pokémon Diamond/Pearl/Platinum games.

Therefore, it's impossible to catch a Shadow Mew in Diamond.