Here's how to catch Suicune:
1. Complete the game. You need to beat the Elite Four and become the Champion of Johto.
2. Find the three Legendary Beasts. You'll need to find Entei, Raikou, and Suicune. Their locations are randomized.
3. Catch Suicune. Once you've found Suicune, you can catch it using Poké Balls like any other Pokémon.
Note: Suicune, Raikou, and Entei are roaming Pokémon. They will continuously move around the Johto region. You can use the "Radio Tower" in Ecruteak City to check the location of the Legendary Beasts.
Remember, Suicune is a challenging Pokémon to catch. Be prepared with Poké Balls, Max Repels, and strong Pokémon to weaken it!