How do you make someone bi?

Answer 1

You cannot make someone something they are not.

Being bi, straight, gay, etc. comes naturally, not by force.

If they do not already have an attraction to someone of their own sex (as well as the opposite since you mention "bi"), you cannot make them have that attraction.

However, there are cases in which people fall in love outside of their primary orientation. There have been gay guys who fall in love with girls, straight girls who fall in love with girls, straight guys who fall in love with guys, and lesbians who fall in love with guys. It's called an "exception", but it doesn't make them bi.

(2) I can't let this answer stand without mentioning the very long-standing theory that we are all bisexual to some degree - I think it was developed by Kinsey. This theory posits bisexuality as the norm for all human sexual behaviour and sees it as a continuum with exclusively heterosexual orientation at one end; exclusively homosexual orientation at the other end; and bisexuality in the middle. It suggests that most people are clustered on the continuum halfway between the middle and one end or the other.

What this implies is that our sexual behaviour may vary at different times in our lives and in different situations but that it is a mistake to think of our sexuality as fixed and unchanging. Many gay men believe that most 'straight' men, given the opportunity and with absolutely no risk of disclosure or discovery, would be willing to enagage in homosexual acts.
You cannot change somebody else's sexuality.