My son, also BF exclusively, could not, for the life of him, figure out the majority of sippys I bought-- some 10 varieties and brands! As the ADA recommends cups with no valve for ideal tooth and speech development, AND there is no "sucking" involved, the two cups that have worked for us are the Take N Toss and the Tilty cup. The Tilty means your child doesn't have to tip their head backward so far to get at the liquid (we adults don't tip our heads back that far when drinking) and the liquid is free-flowing, so yes, it will drip and leak when dropped or shaken, but the idea is to get your kid drinking like a "real" person from a "real" cup eventually anyhow. The Take N Toss are pretty much the cheapest cups on the market (hence the name-- cheap enough to "toss/recycle") but they also are free-flowing (the liquid drip rate is a little slower than the Tilty though).
If your child isn't used to bottles, introducing a Nuby or similar trainer cup that they have to suck on an artificial "nipple" spout is almost like regression to them. It can be tough; my son still drinks a few sips, then he likes to shake the sippy (liquid coming out the top). Obviously we started with water. You also may have to put something your child really wants in the cup-- water is great for them, but in order to really capture your daughter's attention, you may want to try something chocolatey. Personally, I mixed a bit of that organic PediaSmart (it's billed as a 2nd-stage toddler formula) with my breastmilk. He gulped it down and begged for more. The breastmilk made it "familiar" and the chocolate flavor-- well, that just made it all the more yummy. Eventually, you can offer a little of this and a little of that in the cup and your child will have the hang of it enough that they'll at least be willing to "check out" what's in the cup. At that point, it's not about KNOWING how to use the cup, but WANTING to use it. Good luck.