Honestly direct and straightforward “hi, you’re really *compliment* could I have your number or snap?” and if they just laugh or don’t say anything, it’s nothing personal they are probably nervously laughing and just caught off guard so if they say no just be like “that’s alright! Have a good day”
If you are both alone, pay attention to her like if she is wearing headphones or in the middle of something just leave her alone. If it’s dark or an eerie place, don’t approach. Also NO touching, not to get her attention or anything!!! No poke or anything if you think she didn’t hear you, NO TOUCH
If you want to ask a girl out just follow the simple rules: Pause: think if she is in a safe environment to be approached comfortably (is she alone, is it dark, or working at her job) Look: see if she has any universal signs of “leave me alone” like headphones, book, sunglasses and hands in pocket and no eye contact, or if she is in the middle of something like working. Approach: get to the point quickly so she knows why you are talking to her, the longer the pause the more awkward it can be.