You can't. Neither Ho-oh nor Lugia can learn Tailwind.

EDIT: Was wrong. They can get it in official games through Move Tutors.

Ok then, But ho-oh and lugia can learn tailwind in the official games, though not in SV

Double-checked, yeah I was wrong about the official games. Ho-oh and Lugia can learn it in those through a Move Tutor.

But there isn't a tutor that teaches Tailwind in Infinite Fusion, so you still won't be able to.