Are you a SAHM? Is she with you the majority of the day?
If you're going to wean gradually, you eliminate one feeding, say from 5 feedings to 4. Then, after a few days or a week or two, you eliminate another feeding. Usually the last two feedings to go are the first and the last of the day.
If you're using nursing as a way to get her to go to sleep for naps and at night, you probably want to work on putting her to bed awake as well, because bedtime is going to get ugly if she won't go to sleep without nursing to sleep.
I think it might also help to, at first, give the sippy cup in addition to, rather than as a replacement for, nursing. And let her self-feed in general. Messy, but I found with my second child when I let go of the feeding more than I had with my first child, it went smoother. She felt like she had some control, rather than just being spoon fed what I wanted her to have.