Lucy, All I can offer is let her play with the potty for a day or two & see that it's not scary etc... Let her get used to it being around!
Rather than taker her nappy off & swit her on it, as it can be a bit frightening to all of a sudden be placed on something like that!
I know my nephew took ages to train as he was all of a sudden introduced to a potty, with Maddy we had one for her to sit on & play with as soon as she could sit herself!
She was fully out of day & night nappies by about 22months & I think having the potty around from day 1 helped! It wasn't just introduced one day & expected to be sat on for wees etc straight away...
Let her get used to it, even if it means her sitting on it with nappy on for a couple times to see it's OK!

Good Luck, she sounds ready!!!!