The best thing you can do when it comes to strengthening your core after back surgery is to start small.
Start with a basic deep core contraction practice (like this one here) and learn what activating this area really feels like.
From there what you want to do is start adding difficulty to this deep core contraction with basic movements such as leg drops or mini marches. These are really simple low stress exercises that are great for engaging the trunk and introducing the need for what I call Core Coordination.
When you can successfully do this your mind will learn to trust the resilience in your low back and rely on your abs and trunk to do the bracing work instead of defaulting to your lower back. This is a common issue low back pain sufferers have.