Most Helpful Girl
By talking to her. Tell her what made you come to that decision, why you feel you both should break up. Give her closure by answering any question she throws at you.
It's still gonna hurt her very much no matter how you do or say it, but being honest and open with her and explaining things and giving her closure would be the best way to go about this.
I just went through a breakup myself and he was so nice to me; He gave me closure and did everything I mentioned above. Did it hurt any less? No, but at least it puts my mind and heart at ease a bit, and it has made me unable to hurt him, even if I really wanted to. And because of that, we're still on talking terms/remain friends.