Put two or four tennis balls inside a sock and knot the top of the sock. Then, place the balls below the lower back muscles. It's very important that you do not place the ball below the spine — the spine must be between the balls.
Keep your knees bent and feet flat on the floor. To massage the lower back muscles, you can move your hips up and down, and side to side, as well as lift one leg to apply slightly more pressure. Again, you should not feel pain and the moment you feel discomfort, stop.