There are some tips here like how to guide the cup in LOs hands to her mouth and to not put too much in the cup
Transitioning to a Sippy CupIt may also help to put your own drink in a sippy cup to demonstrate and make it fun.
I hear many BF babies prefer a straw rather than a sippy so perhaps try that. I showed my LO how I used a straw and first introduced it by trapping some milk inside by putting my finger on the other end, then with his permission guiding it to his mouth and releasing my finger so a little milk went into his mouth, so he knew what it was.
Or try an open cup?
You might have more luck replacing the afternoon BF with a solids snack and a cup of water, at 1yo the solids meals become the primary food so it's ok for milk intake to drop now.