I wonder WHY you want to let your friend know that you are in love with her, while you know that she's heterosexual.
Tell Her About Your "Recent Lesbian Feelings" First
But when you really want to speak about it with her, I suggest you should inform her about 'recent lesbian feelings'. Not immediately that you are in love with HER.
She May Have The Same Feelings
When she reacts positively, you could maybe tell her that you've fallen in love with her. There even is a possibility that she dares to be honest when YOU are so honest to her. Maybe she never dared to speak about lesbian feelings?? Who knows?
Be careful with anything you say, but do whatever you feel. Follow your heart in this situation.