AIMZ - I would only teach her the sign just before you feed her then teach her finish once she has finished. Here is the link to the Auslan site where you can enter finish into the search bar Auslan - Signbank

Both my parents are deaf so we taught my niece at a young age. By the time she was 1 she could sign many words but didn't quite know what all of them meant. She knew what milk, finish & sleep meant. She was bottle fed so i'm not sure if that made it easier to teach milk. We would ask her if she wanted milk, sleep or if she'd finished, and do the sign for it, if she responded with the sign, she wanted it, if she didn't respond with the sign, she didn't.

I guess you could show the sign, point to your breast, show the sign again, then get her to sign it by controlling her hand yourself.

I hope this helps, at least a little bit. Good luck,it's a great tool to have.