I was half sitting with a pillow behind me, although at physio it was on a hospital bed which you could fix in a half sitting position. My foot was then strapped into the foot rest and off I went!! I started at a lower degree and then during the session I would turn it up as and when I felt comfortable. I managed to move 25 degrees in the first session and only 6 degrees in the 2nd session. The sessions were a week apart.
Hope this helps, and it may hurt to begin with but most physio does until you get your leg working again. Even doing leg slides I had tears rolling down my face and my husband wondered what on earth had happened. Try to feel positive as you know you have to do it to get better, put on some relaxing music, get a bar of chocolate and sit back and let the machine do the work for you!