For a start you need to wear trainers to give a good grip on the floor. Put a cushion or yoga mat down to sit on. get two very small weights such as weights from kitchen scales, mine were 1lb and 8 oz,( I expect this is cheating but this is how I learned to do it, and now I can do it without the weights). Hold weights in each hand outstretched, cross your legs at the ankles, inhale, then lower yourself slowly to the floor whilst exhaling. You should now be sitting cross legged. With your trainers flat on the floor, your ankles still crossed, shift your centre of gravity forwards whilst holding the weights out in front of you. inhale, then exhale, pushing your body forward and up, using your abdominal muscles.
I honestly could not get up at all when I first started to do it, despite doing lots of yoga and pilates etc, now I can do do it with relative ease. I am thinking of making a you tube video of it, but no doubt there are loads of them already out there.