A place to start is your standing position.
Inhale bringing your belly button to your spine.
Ribcage tipping back toward the spine.
Torque and Trunk Stability Part 1: How to Stand

To help lengthen and strengthen work on
Planks: have body horizontal to the ground butt in line back straight.
Start with short intervals and build the time in plank position.
Do 3 or 4 at a time. Take some rest between attempts.

Crunches: with legs in the air.
Lunges: