go to prof elm lab and get masterball
go to eruteak to fight the kimoni girls.(5 of them)
get the bell from them
you should have the rainbow wing
you should have defeated team rocket in radio tower
go to tin tower after you have masterball,bell and rainbow wing,defeated kimono girls
talk to kimono girl at the upper most lvl,ritual starts and ho-oh will come.
this is a 1 chance battle.. fail to catch it and it will be gone forever ...