Just be like "Hey if you aren't doing anything today do you wanna go grab a bite to eat or something?"
It depends... Do you know this person or would it be the first approach?
Asking someone out might be scary at times, especially because you might be scared of a possible rejection but don't be! You've got nothing to lose.
Just be simple and direct. If you meet her you can start a conversation and then ask her if she'd like to grab something like a coffee or a bite to eat like Izzy said.
If you don't have the chance to meet this person, you could also text her or call her (is not that scary as it sounds), ask her how she's doing and then ask her if she'd like to go around and maybe have a cup of coffee with you.
Simply just ask her naturally. Do not listen to inner voices preventing you from having what might be a really nice date.
Best of luck! (: