Adjust the padding to be at your mid chest when seated straight. Then sit down and hook your feet under the foot pads and grip the handles securely.
Start the crunch by activating your core and hinge your torso forwards as far as comfortable, then slowly return to the starting position. Keep your lower back constantly against the padding. Repeat for the desired amount of repetitions.
Avoid using the arms to pull the weights down or rushing repetitions.
Fitness for all, for life.