Im a trainer i train bigger horses this ones a mini I've worked them but i know what to do now.
Put a saddle on and maybe put a dummy that is shaped like a person on the horses back ? It sounds stupid but some people do it when they break horses in
To be fair, if you have to ask that question, it's probably safer to get an experienced rider in and watch them do it.
Gain trust through ground work first and then start backing by leaning on the saddle and get an assistant to hold her. If she is stubborn it will take a long time.