Summarize this article:

Bend your knees and keep your feet flat on the floor. This stretch is great for people who already have lower back pain. It stretches the lower back along with supporting muscles in the hips and glutes. Keep your other leg in a comfortable position - usually against the floor, either straight or bent at the knee. To add a hip-flexing element to the stretch, use your hands to rotate your leg by gently pulling your shin across your body.

Summary:
Lay on your back on a carpeted floor or exercise mat. Hold the leg against your chest for about 30 seconds.