What you do not want to do is get in any kind of dramatic argument where there is blame going on between parties. This will only bring bad emotions your way which you don't want. Someone with NPD always wants to be right, so you have to learn to just let them have the last word and shrug things off even if you feel hurt. Take the high road, be brief, thank them, and say you're just doing what's right for you and tending to your life needs. Wish them the best and go on your way. Leave the place clean. Don't feel obliged to do any special or dramatic things before you leave. Just tell her you want to have a "chat". Start off complimenting them on what you've enjoyed about staying with them. ("I want to talk about a few thing...so I've really enjoyed you as a roommate, and I like x y and z about you.) Then insert your desire to leave. ("With that being said, I feel I am being lead to a new season in my life and will be moving out soon. My last day will be July 31st." Then end with another compliment / happy farewell. "Just wanted to give you this heads up, thanks again for being an awesome roommate".

Obviously I'm extremely vague in my wording so you can be more creative. But the point is, be kind, be direct, make it brief, and close the discussion on a good note. You're not making elaborate dinner plans, or some kind of farewell speech. You're just saying, hey I enjoyed my time here, now I'm moving out, thanks for being my roommate.

Hope this helps you.