I would say first try to become part of her usual world, you know, try to be seen and heard by her but not too near, just be something usual for her, so that she finally thinks you are harmless. How do you say that, you know, desensitizing her to your presence?
Once she does not mind your being close to her, talking, moving around, there will be some good thing done I think.
I knew a mare, who had been left alone in a pasture for years and had a laminitis (? fourbure in French), she was in deep pain and her owners didn't know anything about horses and did crazy things with her... I ended up trying to help her 'cuz she became dangerous... My approach was: as long as she looked defiant, I didn't try to get closer. When she looked a bit calmer, I just stretched one hand forward, pretending I was stroking her but not touching you know? It looked quite ridiculous I know, because I also kept murmuring some soothing lullaby to help her relax, and moving my hand as if to stroke her but at one yard distance, etc. It took me a long time... Because whenever she took a nervous, angry or frightened posture, I stepped back, really trying to tell her "hey calm down, I'm not invading you, you just decide, if you accept me, I come nearer, otherwise I'll just be waiting here for you to be ready. Just calm down, breathe...". She gradually understood I would not kick her or force her into any unpleasant thing, and as I was always moving my hands calmly around her and nothing happened to her, she gradually allowed me to draw nearer... Till I could touch her shoulder, really slghtly and briefly. She started all the same so I immediately drew back a little, let her relax, and so on.
It's long, it seemes ridiculous, but she was really in pain, and afraid, so I thought I had no chance to get to her by "force" or "speed", and I just tried to show her I was not a monster. By doing this several times, I finally could take her feet and touch her legs to try and see where the pain came from. That's just an example.
I still don't have the words to describe the postures but I guess you can see whether a horse is relaxed and tolerant or nervous and ready to kick you out!