Instructions
Earn the seventh badge from Cianwood City. Return to Goldenrod City and defeat Team Rocket to earn the Rainbow Wing item.
Return to Ecruteak City and enter the Tin Tower.
Proceed to the top floor of the tower, where you will find Ho-Oh waiting on the roof.
Save your game prior to engaging Ho-Oh, as it̵7;s the only one in the game and if you lose the fight you̵7;ll never be able to catch it again. Engage Ho-Oh by walking up to it and pressing the "A" button.
Use a team of Pokémon (level 40 or higher) to battle against Ho-Oh. Use electric attacks to paralyze the Ho-Oh or sleep-inducing attacks to put it to sleep. Use an Ultra Ball or Great Ball to attempt to capture Ho-Oh when its health is low (in the red) and it is asleep or paralyzed. Repeat until you successfully capture the Ho-Oh.