Help with asking a girl for her number

Ok so here me out. I’m thinking about asking a girl for her number tomorrow, but I need a bit of advice.

We used to know each other 6-7 years ago a bit but it’s kinda fallen off, and although she likely remembers who I am, she doesn’t actually know too much about me. Tomorrow is my last day I’m able to ask her out before a long break that I am going on, so I need to do it fairly quickly.

The only time I’m able to talk to her is in the halls at school because of our lack of classes together, so we will be walking after class trying to get to our next class, so I can’t talk for too long with her.

I haven’t rehearsed anything but I do have a general idea of what I want to talk to her about. I don’t want to ask her for her number right away, but I have probably 2-3 mins before we have to get to our classes.

Any advice is appreciated.