DO NOT tell her straight up that you want to have sex with her. It might work but it's high risk.
Go out with her, and imagine that she's just a girl that you've just attracted and need to build rapport with. Tease her, neg etc. Don't over-compliment her or she'll put up her ASD as she'll believe that you're just being nice to get in her pants. Don't insult her either because she might be getting weary of you (due to your history with her) and may think that you're just leading her on.
In short, don't act needy and don't be a dick

should be pretty easy, GOOD LUCK!