I am going to say the same as everyone : be really calm and gentle with her. She has lived and seen tough things, so she will probably have some "weird" reactions that you don't really expect or understand at first. The point is not to be weak and let her do anything, but to find the right balance between being gentle and solid. Your mare must understand that you respect her and that she can rely on you, and then she will trust and respect you too. Maybe groundwork would help to build this trust.
I advise you to find a trainer or professional to supervise you, so that you are sure to do the right things, but choose well your trainer, and make sure that he respects the fact that your mare came from a neglect situation.
Good luck with your mare ! If you can, keep us updated on this post, so that we know how things are between the two of you :-)
Take your time with her. If she is showing any signs of stress or discomfort take a step back on what you are doing and go back towards the basics. Dont force her to do things she may not want to do or she is not ready to do, such as jumping or dressage. It takes patience.
i agree with Kate! before you even try riding or training, work on building a relationship where she trusts you. you can’t get anywhere if she doesn’t have total trust in you!
just be patient and kind. if she’s skittish, give her time. once she learns to trust you, then move on to actual riding or more intense ground work