If you want to wean your baby gradually, simply nurse her less often and for shorter and shorter periods of time. If you're nursing your baby three times a day for 20 minutes at a time, you can start by nursing just twice a day for ten to 15 minutes at a time for a couple of days. Then you can keep reducing the amount of time and frequency of nursing even more until your baby is completely weaned from the breast. By doing this, you'll produce less and less milk, and that will make weaning more comfortable for you. It will also make weaning more pleasant for your baby, since she'll be progressively adjusting to nursing less and drinking more from the sippy cup.
Of course it's entirely possible to just stop nursing your baby today, and some moms say that's the easiest way, at least from an emotional standpoint. If you do this, you'll probably have to contend with some breast fullness and discomfort, and it might help to pump a little to alleviate that discomfort. You might also have to contend with your baby's cries. But it's okay to stand your ground! At this point, weaning your baby is best for everyone involved—you, your 15-month old, and your unborn baby.