1. Get the Odd Keystone.
This is located on the third floor of the Old Chateau in Eterna Forest, which can be accessed after earning your eighth gym badge.
2. Head to Turnback Cave.
This cave is located on the north end of Route 214.
3. Use Strength to push the Boulder.
At the south end of Turnback Cave, use the Secret Key (obtained from the Old Chateau) to unlock a door, and then use Strength to push the boulder inside to open the path to the Distortion Room.
4. Enter the Distortion Room.
Inside the Distortion World, you'll find Giratina's maze. The maze is constantly shifting, making it challenging to navigate.
5. Follow the footprints.
To find your way through the maze, follow the footprints on the ground. These will lead you to the center of the maze, where you'll find Giratina.
6. Battle Giratina.
Once you reach Giratina, you'll need to battle it. Giratina is a challenging opponent, so be prepared for a tough fight.
7. Catch Giratina.
After defeating Giratina, you'll have the opportunity to catch it. Use your best Poké Balls and try to weaken it as much as possible before attempting to catch it.