Instructions
Travel to the top of the Bell Tower once you have defeated Team Rocket and the Kimono Girls and obtained the Rainbow Wing. The Bell Tower is in Ecruteak City.
Go up to Ho-oh and press "A" to begin the battle.
Use water-, electric- or rock-type Pokemon to weaken Ho-oh. Do not use ground-, ice-, plant- or steel-type Pokemon, as Ho-oh has powerful fire and flying attacks that can knock out these Pokemon with one hit if they are around the same level.
Use a Pokeball to capture Ho-oh once its health is in the red area. Ho-oh has a very low capture rate, and this may take several tries, so come prepared with several Pokeballs. Put Ho-oh to sleep or paralyze it to increase your chances of capturing it.