If you want to maximize the results from your crunches and avoid injury, the key is proper technique!

The Basic Crunch
Lie on the floor on your back with knees bent, feet flat on the floor about hip-width apart. Place your hands behind your head or neck for support. Keep your chin pointing upward while you lift your head and back up, make sure your chin isn’t against your chest. Crunches are not full sit-ups so you only need to sit about 1/3 of the way up each time. Keep your back and your neck in a straight line and use your abs to lift you, don’t pull on your head with your hands, (remember, they’re just there for support)
Pause in the raised position, and then slowly lower back to the floor. Each crunch should be done slowly, taking 4-5 seconds to really target your abdominal muscles.

Try to aim for 3 sets of 15 crunches in the beginning. As you build strength in your core you can work your way up to 25+ crunches per set, or try different variations of the crunch to target different areas (there are plenty to choose from!) Be sure to exhale as you crunch up and inhale on the way down. Good luck and happy crunching to you!