This lower back exercise is useful in itself, but also prepares ground for the other lower back pain exercises. Please do this one first, every time, before you do Rock on Hands and Knees, Gentle Leg Movements orGentle Pelvis Rotation.
An alternative, seated version is Find Sitting Bones.
(You might want to lean against a wall.)
Gently press a couple of fingertips into your side at buttock level. Raise and lower your knee. Gently prod with your fingertips until they are touching hard bone and they move a little as your knee moves. You're now touching part of your hip bone.
Now find the other hip bone in the same way. Leave your fingers where they are for two minutes. Remind yourself regularly during that time that this is where the bottom of your spine is. Keep your eyes open: you can look after your body and live your life at the same time!
When you take your hands away, this lower back exercise should give you a clear sense of your hip bones, and a feeling that your fingers are still touching them.
That feeling shows you roughly the level at which your spine ends: