It can go two ways.
You say, 'Pardon, Ive seen you here before and was wondering if i might sit with you?'
What happens:
If you're attractive, she says yes and begins to tease her hair, talk about her body/whatever to bring attention to her nice features.
If you are NOT attractive, she'll scowl at you, tell her friends you are a creep and avoid you from then til death.
Girls WILL admit this is what goes on ALL DAY. The same joke out of two guys mouths, one average or less, the other above average looking, who you think is goin to be the 'funny' one?
Solution if you are unattractive:
Acquire cocaine. Share cocaine with said girl. Receive pussy. Post pics in Smash or Trash.
Side note, let girls see you with other girls. It deffo will up your cred with them. Theyll be thinking 'well if she is fuckin with him, there must be something good goin on there.' Ive seen it happen, dudes getting ZERO love from ladies, but once one girl says 'oh hes fine honey' then theyll all jump on board. Serious.