Abdominal crunches are an important movement to learn and implement for improving abdominal strength.
Lay flat on relatively hard surface. Bend your knees and have your feet on the floor. Cross your arms over your chest. If your neck is sensitive, hold the back of your head. Note the higher your arms are the heavier your body will be for you abdominals when doing floor crunches; arm placement does matter.
Slowly begin curling your back off the ground, pulling your ribs down towards your pelvis/waist; your low back should maintain some contact with the floor. The end goal is to fully round your spine by contracting your abdominals. Hold for a second at the top and then slowly return down; control both directions and don’t use speed to help you through the movement.