How to ask out a girl you have known for a while?

We'll it's pretty easy. Just act natural. Ask her out on the way you would be talking to a friend.
Do not ever express how important is the future date you're planing.
If you show too much affection at the very first time you can easy creep her away.
Take it easy, just randomly ask if she'd like to have a coffee with you,
next time make it to a dinner, cinema, weekend away... ;).
She'll know there is a reasn you are hanging out so much.
At one point you'll feel quite comfortable holding hands, kissing, etc...
It sounds a bit old-fashioned but sure it works fine.