So you raise some ferrets. And you give one the flu.
Let it infect another. Let that one infect a third.

Continue this serial passage until a ferret in the same room, not just cage, gets sick from flu.

Now you have an airborne variant of the flu you started with.

For sci bonus points, sequence it and comment on mutations.

For Streisand effect, have the USG try to censor your work.