A carpeted floor works great. Imagine that you're being pulled from your feet and the top of your head.  This lengthens the spine. Tuck your head slightly toward your knees.  Try to make your lower back curve out as you do this. Move softly, avoiding hard and jerky movements.  Don't rock up onto your neck.
Find a flat surface, preferably with light padding. Lie on your back and stretch. Tuck your knees into your chest and wrap your arms around your legs. Rock gently up and down on your spine, slowly increasing your motion until every part of your back (except your neck) is rocking on the ground.