First, warm up your glutes and lower back, two underrated keys for complete core development.
1 Cliffhanger extension
Lie face-down on a bench, legs hanging off it, toes touching the floor; this is the start. Keeping your legs straight, squeeze your glutes and try to raise your legs so they’re parallel to your torso. Hold for 6 seconds and return to the start. That’s 1 rep; do 10.