The best way is to get some codes online. Using an online forum is a great way to do this.
Chat with our AI personalities
first manaphy has to tell you about the marine resort. second you must have a secret slab or a mystery part in you bag you do not have to use or hold them then go to dark crater b6f and keep fighting him until you recruit him
Actually, at Undella Bay, there is a place where you can go in the ocean, and in that place there is a dark spot in the water. Your pokemon has to know dive, but you can dive into that dark spot with dive. Once you do, you will see manaphy's castle, once you go in, you will have to fight it and capture it
you go to the third gym you go up you go to the grass and you randomly walk around and voila! dark lugia if you want shiny do the same thing but release your mew and mewtwo and there is shiny lugia.
Geodude, Dunsparce, Zubat and some water pokemon.
Darkrai is a dark type of Pokemon. You can catch one in Pokemon Diamond version on Newmoon Island but you need a membership card.