Exactly what I've been saying: if a girl or woman wants to make her best friend her lesbian lover, first try telling her she's had sex with x number of girls. She'll ask you how it was, to which her seducer tell her "great, way better than any guy." Since every girl has wondered about it, maybe even masturbated to orgasm fantasizing or watching lesbian porn, she'll either ask her seducer if they can do it, or say "yes" when she suggests it. Done. She'll be hooked and the sex from then on will take their friendship to a whole other level, maybe even to marriage.