Rest her arm on her chest. Then line up the long edge of the sling along and below it, so the point of the triangleis by her elbow, and the long thin pointy bit is over her shoulder. Tuck the straight edge under her arm; then bring the other long pointy edge round her back, and tie the two points together on her shoulder. Tuck/pin the other point in by her elbow.
I think that's right, It's a long time since I was a nurse!