It's always ideal to start with a good stretch, and knee to chest is one of the perfect stretches to start with especially for lower back pain.
Lie on your back with both legs extended on the floor.
Keep one leg extended and touching the floor while you bring the other knee up into your chest.
Pull your knee close with your hands and hold it for 15 to 30 seconds. You shouldn't pull or hold so tight that it's uncomfortable. You should feel stretching in your glutes and lower back.
Lower the first leg and repeat with the opposite leg.