This is one of two exercises I recommend for all my patients with low back problems. The other is the Foundation exercise http://bit.ly/2iAMTHf .
Go to the sporting goods section of any store such as Wal-Mart, Fred Meyer, Big Five, etc.. You'll find a chart on the side of the box containing exercise balls. This chart tells you the correct diameter ball to buy for your height. The ball comes with a hand pump for inflation. You can also find them online at http://amzn.to/2w01BK8 .
When you first begin using your ball, stay close to something to help maintain your position on the ball. It can tend to "throw you off" to one side or the other until you get used to it.
Position yourself sitting on the floor with your hands holding the ball behind you. Push slowly to begin lying back over your ball. Continue as far as you comfortably can while keeping your head in contact with the ball. If your head loses contact with the ball, you've gone too far. Roll back towards your feet to keep your head touching.
Begin with 5 or 10 seconds and progress comfortably to 30 seconds. This may be repeated as many times a day as you need to relax your back muscles. It's especially good to do upon arising and before bed.
If my back is bothering, I may do this, and the Foundation Exercise, dozens of times a day. Usually, within a few days, the back pain is gone.