I had some fairly bad back issues where I had to rebuild my core from near scratch. I started with some light but steady core exercises, knee slides, leg lifts, bridges. Started very low and slow, and built up. Just 5. Just 7. Just 10. And so on. Stopped doing the exercises that made pain worse, concentrated on others until I healed enough to expand repertoire.

The key was habitual repetition, every morning before I got out of bed, every night before going to sleep. The reps lay the groundwork and increased range and strength.

Try not to overreach. Re-injury slows down the whole process. (Sometimes this is the hardest thing, especially if you have a competitive nature. Competing with yourself can sometimes be counterproductive.)