Lie on your back, one leg stretched , the other bent at a right angle at the hip and knee joint.
You can then help guide the angled leg across the stretched leg ( your hand can support the stretch ). Turn your head in the opposite direction of the angled leg. Perform on both sides.

This exercise is also good for stretching your spine.
Ensure both shoulders are flat on th floor , arms out stretched , keeping both knees bent and together.