first off if you have any back issues you may want to find a core exercise that has less trunk flexion than the traditional crunch. continuous curling of the spine can deteriorate the disks in your back.
Start by pushing lower back firmly into the mat slowly lifting shoulder blades off the mat by curling your trunk pushing your hips towards your chest.
Move very slowly try to hold at the top of the Motion, and slowly lower yourself down, keeping lower back pressed into the mat through the duration of the exercise.
Cheating this exercise can occur by using momentum and quick movements through the motion. Move slowly enough and hold long enough; and you will feel your abdominal muscles start shaking in the confusion of how they're supposed to be working.
Do not lock your fingers behind your head place fingers by temples this will prevent you pulling your chin towards your collarbone and stressing your neck.