http://www.freetrainers.com/ crunches will not help your lower abs. Law down on your back with both legs towards the ceiling. Point your toes downward. Start by using your hands as support, and try to push your heels towards the ceiling. Try not to rock your legs but instead focus on your heels by pushing upward. This is often called a reverse crunch. Focus on doing the movement slow and controlled. Do not hold your breath. It won't do much good if you do. Once you get comfortable with this, try doing a crunch at the same time. This time, bend your knees, but with your knees still facing upward and bring your knees up toward your chest. This should help. There are other exercises that help with the lower abs such as keeping your legs straight and together and bringing them slowly towards the ground while lying on your back. Pull them up quicker and slowly let them down. Don't let them touch the ground. You can then try doing it with your legs spread apart.
I hope this helps and wasn't too confusing. The above address is useful and shows pictures of each exercise.