Step 1: Ensure that you have beaten the Elite Four and Champion Lance in the Pokémon League.
Step 2: Go to the Tin Tower, located in Ecruteak City. Inside the tower, climb to the top floor and interact with the Burnt Tower. After a cutscene, Ho-oh will appear.
Step 3: Initiate a battle with Ho-oh. Be prepared for a challenging fight, as Ho-oh is a powerful Pokémon.
Step 4: Reduce Ho-Oh's health while trying to avoid knocking it out. Lowering its health will make it easier to catch. It is also recommended that you use status inflicting moves like Paralysis or Sleep to help you capture it.
Step 5: Use an Ultra Ball or Timer Ball for the best chances of catching Ho-Oh. Ultra Balls are effective for catching wild Pokémon in general, while Timer Balls become more effective the longer the battle lasts.
Step 6: Continue to battle and weaken Ho-Oh until you successfully capture it. Remember to save your game after catching it to ensure you don't lose your progress.