1. Have a yoga strap nearby.
2. Release your left leg flat on the floor (you may keep it bent with the sole on the floor if this is more comfortable) as you draw the right knee in to your chest.
3. Loop the strap over your right foot and straighten the leg coming in to Reclined Big Toe Pose - Supta Padangusthasana.
4. Repeat on the other leg.
Flat Low Back Posture from Back Pain Guide Anne Asher.