Instructions
Travel to Victory Road, which is on Route 26, and pass through it. This will lead you to the Elite Four.
Talk to each member of the Elite Four. They will challenge you to a Pokemon battle.
Defeat each member of the Elite Four. The first member, Will, uses psychic-type Pokemon. Use bug- and ghost-type Pokemon to defeat him, such as Gengar or Beedrill. The second member, Koga, uses bug-type Pokemon. Use flying- and fire-type Pokemon to defeat him, such as Fearow or Charizard. The third member, Bruno, uses fighting-type Pokemon. Use psychic-type Pokemon to defeat him, such as Alakazam. The last member, Karen, uses dark-type Pokemon. Use fighting-type Pokemon to defeat her, such as Machamp.
Defeat Lance, who will challenge you as soon as you defeat the Elite Four. Lance uses dragon-type Pokemon, which are more-easily defeated by using Pokemon with ice-type moves, such as Articuno or Gyarados.
Travel back to the Bell Tower, which is where you first fought Ho-Oh.
Travel to the seventh floor of the Bell Tower and go up the ladder to the very top. You will see Ho-Oh standing on top of the tower if you have defeated the Elite Four.
Press the "A" button in front of Ho-Oh to challenge him to a battle. Use your Pokemon to attack him and lower his health. This will make capturing Ho-Oh easier, as he will be less likely to escape from Pokeballs.
Throw Pokeballs at Ho-Oh until he is captured. This may take several attempts, as Ho-Oh can break out of Pokeballs more-easily than can other Pokemon.