Simple answer: You don't.
Longer answer: No matter how you play this, she will get hurt. She is in love with you(or loves you), and being told that someone you have those feelings for doesn't want to be with you hurts like hell.
However, the way in which you break up does help a little. If you want to stomp her heart to pieces, you rub in all her bad habits and how you hate them. If you want to break her heart gentle, you put the blame with yourself. How you twist it, doesn't really matter, as long as you make it sound like that you are not good for her. (Notice how I left out "good enough", because that would pedestalize her)
I dumped a plate who was madly in love with me by telling her how dominant I am and that I would crush her and push her into a corner. That she needed someone more gentle than me. Still hurt her feelings, but at least she didn't have to hear how clingy/needy and submissive(in a bad way) she was.