Ok so let's see on a tutorial:
HOW CAN I SIT FROM INVERTED STRADDLE!?
2. Now the tricky part, it is good if you can swing yourself as much as you can tooo....
3. Catch hoop with your hands AS HIGH AS YOU CAN! The higher you'll catch, the easier will be getting into basic sit! You can try streighten one leg while doing that, it shoudl help you. And now: PUSHHHHHHH!
4. You're in the basic sit! :)
Don't be worry if it won't come easly for you. It require a lot of strength so be patient and finally you'll do that, to preper yourself to that you can do crunches while hanging on your underknees. :) Like here: