If you've decided it's definitely what you want to do, just gather your courage and do it. The way that Intrigue suggested is good. Not that anything makes it easy but his way is probably least hurtful.
Whatever you do, don't use the method I did with one of my relationships. I didn't have the balls to break up with him to his face, so I cheated on him in the hopes that he would break up with me. I ended up feeling awful because I think cheating is wrong, and it didn't work (he forgave me), so I had to get that courage together in the end anyway. Big, big mistake. In my defense, I was young and drunk. It was still a bad way to do things.
If it isn't something you think you can live with forever, I do urge you to move on sooner, rather than later. The longer you wait, the more likely you are to end up feeling as though you wasted the years you were with her.