If you have access to a balance ball or can find one to purchase, this works well for me:
1. Sit on the edge of a chair with your back straight and cross your legs so that your right ankle is on your left knee.
2. Place the balance ball under your right knee.
3. Press down gently on the interior of your right knee while holding your ankle in place with the other hand.
4. Repeat for the other side and, if needed, do another set leaning forward slightly into the stretch.
Just be careful not to force it. The balance ball provides some stability and resistance, but it is easy to overdo this one. Good luck!