Adjust the seat height so that your shoulders are against the top of the padding. Hook your feet under the foot pads and place your elbows against the padding whilst you grip the handles.
Start the crunch by activating your core and bend 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 having the lower back come off the padding.