The first place I would start is the core. Engaging and contracting the deep core involves contracting the muscles in the lower back.
Learning to engage this area and turn on your deep core is step one to a solid lower back exercise routine.
I demonstrated how to do this in today’s video.
The next step is finding a low-threat exercise you can do, preferably on your tummy, to activate the muscles in the lower back without too much stress or fear.
For this, I usually cycle between the:
Lower Limb Only Single Leg Raise

Opposite Arm/Leg Raise

Upper Body Raise Only

You can do these as small and gently as you want or make them harder and more intense. It’s up to you to pick the ideal level of stimulus for your unique situation.