Tell her exactly how you feel. Let her know how much she means to you and how much you have enjoyed your time so far together. If you have feelings for her, then you need to tell her. Let her know that you do not want to see anyone else if you want an exclusive relationship with her. Don't let her second-guess your feelings or have to wonder what you are thinking. Tell her. The truth is the best way to let her know how you are feeling and which way you want the relationship to go.
Time the question right. Don't ask her while she's in the middle of her morning jog or while she's scrubbing floors or washing your clothes. Take her out to eat or cuddle up on the couch for some alone time. Go to a favorite place that you enjoy together or do something special and watch her favorite movie with her. Make the moment romantic and special to her.
Take your time and don't rush the question. Make sure she's looking in your eyes and ask her slowly. If you rush and slur your words, she might not even realize what you are asking. This is an important moment. Take a deep breath and make the moment count. You will appear more confident and sure of yourself if you are calm.
Use your imagination and come up with fun ways to ask her to be your girl. One way to ask her is to get down on one knee to tie your shoe and look up at her and tell her that while you're down there you should go ahead and ask her to be your girl. Another way is to write her a cute poem asking her to be your girlfriend. Put it on her doorstep and leave a note for her to call you when she decides. You also could spell it out with alphabet cookies and hand deliver them.