Here's how to catch Ho-Oh in SoulSilver:
1. Complete the main story: You need to have finished the main story and defeated the Elite Four.
2. Find the Rainbow Wing: This item is found in the Ilex Forest, in a house behind the Gym.
3. Travel to Ecruteak City: Head to Ecruteak City and talk to the old man in the building next to the Gym.
4. Go to the Tin Tower: The old man will tell you to go to the Tin Tower. You'll find Ho-Oh on the top floor.
5. Prepare for a tough battle: Ho-Oh is a powerful Pokémon, so be sure to bring your best team and stock up on Poké Balls.
6. Catch it! Use your best Poké Balls and hope for the best! It might take several attempts to catch Ho-Oh.
Remember, Ho-Oh is only available in Pokémon SoulSilver, not HeartGold.