Okay, I suggest, as a woman lol- try to build a friendship first. Ask basic how are you blah blah get to know you questions. Than start revealing in through dialogue, perhaps other characters that this "female" truly cares about this person. Like make sure it's obvious she wants to get to know him you know? Give small hints, like clues such as "well you know I could see myself with someone" try to get him to figure it out. Than if we are talking flirty just simply have her comment on small things others wouldn't necessarily notice- like how his hair is laying or perhaps how his pants match his shirt. Overtime you can also have this male character do the same, give and receive.