Really, there's not much to it, you either ask her or you don't. Just mention it casually, and more importantly, be calm about it. In my experience, if you sound nervous about it, they might find that less appealing somehow (don't ask me why, I'm a guy 😛). She'd be more likely to go with someone who sounds confident. And most importantly, just don't worry about it, if she's a nice girl then she'll be nice about it even if she says no.