First make sure she likes you. This can mean she's flirting with you or she has told you. If you are not sure you can always ask her best friend.
Second get her alone or out of the crowd this makes it easier for both you and her.
Then ask her if she would like to go somewhere on Saturday or along those sorts of lines. There is a 30% chance she will say yes. Not great but good enough.
If she says yes great you are away.
If she says no then just say its cool don't start crying or run off stay strong.
