How To Get Cresselia Legendary Pokémon (BDSP)

First you need to finish the Pokémon league and register all 150 Pokémon to the Sinnoh Pokedex.

Then you will have to get the national Dex and the poke radar from the professor by talking to him in sand gem town.

Now you need to go to Canalave City.

Click Town Map option.

Now talk to the NPC in the house with the sick boy.

Next you will have to talk to the NPC next to the boat outside to go to Fullmoon island.

Select Yes! to go to full moon island because you need to get the lunar feather.

You have reached the Fullmoon island!

Here, interact with Cresselia Pokémon.

And then pick up the lunar feather.

Now you need to go to Jubilife City.

Click Town Map option.

Enter in the poketch company building.

And talk to the president to get the marking map app for the poketch.

The marking app allows the placement of markers.

Open the app and keep changing areas in Jubilife city Route 203, until the legendary pokemon is in the same route as you.

Once you are in the same route as the pokemon, save the game and walk into the grass.

Click the Pokémon option and select Save your progress.

If you are lucky you will encounter Cresselia, this Pokémon will flee right away so you will have to get the Pokémon hp down a bit at a time since all dmg and status effects will stay applied when you meet again.

Push Luxray! to tackle this pokemon.

Use a quick ball to catch Cresselia!

Your Pokémon gained Exp Points!